I get the impression that many GMs are afraid of running Spelljammer because of the rules about ships and gravity planes and crystal spheres and air capacity and naval combat, rules which have not officially been updated since 2nd edition. You can actually skip all of that, especially at low levels. Here is how I changed the setting for easy fun play that basically any GM can run (I am assuming 5e, but most of this is generic enough to be easily adapted to 3rd or 4th or Pathfinder):
Basic Rules (Setting Intro)
There are no stars or planets as in our universe. It is a big zero-gravity zone of breathable air with lots of things floating in it, including islands of various sizes and ecologies, rivers coursing through space like veins, derelict ships and abandoned fortresses, floating forests anchored to nothing, chunks of elemental matter, giant creatures floating lazily around, stormclouds of magical energy, rips in the fabric of space that lead to other places of existence, skulls of dead forgotten gods, etc, etc. The sun is a thing the size of a large ship that flies through the sky every day, and stars are boulder-sized chunks of elemental fire.
Gravity works differently. Space has no gravity. The floating islands make gravity, but only in one direction and only out to a distance about equal to their diameter. Adventurers dock at the side, just like landing a sailing ship at an island, and then walk around on it. Most ships cannot stay up in gravity, so they cannot fly too close over an island. Ships also makes gravity so people can walk on the deck.
Until high levels, all naval combat is basically just a normal combat encounter, with random monsters attacking the ship, or pirates on fast unarmed ships trying to board, and the PCs defending their ship like it was a fort. Just keep the ship on the center of the combat grid and place everything relative to it as enemies move to keep up and intercept. If you want, and it is appropriate, you might describe the action as a chase scene with everything moving and twisting in three dimensions, but this does not affect the grid, because all locations and velocities are relative to the ship.
That is all you need to start. Introduce the setting by having the party work with an NPC who owns and flies a small ship and wants their help with something. Describe them sailing through a sky filled with tantalizing wonders, moving from one exotic locale to another. Over time, as the party levels up and learns more about the setting, they will want their own ship, and you can start to introduce some more rules:
Distance, speed, and mapping are handled abstractly. All distances are defined, both on charts and in common conversation, in terms of a number of days of sailing. For example, "Crystal Island is three days sail from Gertrude's Rock." This convention developed because most ships travel at about the same speed, and it is difficult to measure distance or define locations in 3-D space. Maps and charts tend to be abstract and sparse, typically centered around instructions for traveling from one point of interest to another. The travel time to a place might be different than the travel time back, because of the prevailing winds or other conditions.
Ship Designs and Costs
There are a wide variety of ships designed for adventuring parties, which will move a party, a couple NPCs, and supplies between islands. Each ship is defined by one number: its range. The range is the number of sail-days travel (one-way) the ship can safely go. If the crew exceeds the range without resupplying and resting off the ship, they have to pass Con checks each day or suffer exhaustion. Range is based on the ship's cargo space and facilities, and larger ships are required for longer ranges. The cost of a ship is 50 gp times its range squared. All other details are cosmetic and up to your imagination.
For example, the 50 gp range 1 ship is a flapboat, which is like a rowboat with wings instead of oars. It also has a little mast and sail. It has no facilities, just room for the party to sit down, and no place to sleep. Low-level parties will use them to move to neighboring islands less than a day's travel away, and they will have to camp on those islands. If the destination has no place to rest, and they will be adventuring on it, they can only go to a place a few hours away.
By level 5 or so, after finding their first realm-tier (challenge 5-10) treasure hoard, a party should be able to afford the 5000 gp for a ship with a 10-day range (or at least the 3200 gp for a range 8 ship). Alternately, they might take it from a defeated raiding party, or be awarded it after a quest, or find and repair a crashed one.
Obtaining a ship directly replaces a treasure hoard. At higher levels, many or most treasure hoards will come in the form of seized ships. Reduce the money amount by the sale value of the ship; they will get the cash after selling it.
The range-10 ship (and ships with a similar range) will be like a 40-foot yacht, with a small galley and private bunks to sleep in. It will have storage capacity for 10 days of food, water, and supplies, including spare rope and sailcloth and tools to make minor repairs. It can be used to go to an unexplored or barren destination up to five days away, although a wise crew will limit such trips to places three or four days sailing away, in order to account for unexpected stops or emergencies.
You can choose one of two options for the longer-range ships. If you want a lower-magic campaign that more closely resembles historical conditions, then make them larger to hold more supplies. They will be harder to handle, and will require more crew members to move the larger sails and wings. Their list price includes the cost of hiring, training, and provisioning the additional crew.
I prefer the magitech option where the adventurer ships with a range of 15 or more are faster rather than larger, because they are equipped with something like a spelljamming helm or an elemental engine. In this case, a range 30 ship costing 45,000 gp might be about the same size as the range 10 ship, and have ten days of food and water, but be equipped with something that lets it travel three sail-days distance in only one day. At your discretion, this increased speed may or may not not allow for faster tactical movement; it is possible that the ship still uses wings and sails for maneuvering but turns on the magitech for longer straight shots.
Each sail-day traveled costs the party 1 gp of supplies, including food and miscellaneous supplies to fix (or fuel) the ship. When the party gets back to port, they have to spend 1 gp to resupply for each sail-day they traveled, in order to restore the full range of the ship. In the case of faster magical ships, this cost is based on distance traveled, not time spent.
Ship Weapons, Upgrades, and High Level Combat
In a medium to high magic world, mundane siege equipment is simply not worth putting on a ship. Their weight, and the extra reinforcement required to mount them, requires a heavier and more expensive ship that will require a larger crew to operate. It is far better to have the artillery be magic-users, and almost anyone who can afford to build a battleship loaded with ballistae can afford to hire magic-users and buy flying mounts or small fast ships for them. These kinds of forces make nonmagical warships obsolete in the same way that naval aircraft made battleships obsolete. Also see my previous post on adventurers making conventional military obsolete.
In a high-magic world, the party will be fighting enemies like them, so high-level naval combat will basically be a showdown against powerful NPCs using normal combat rules in interesting terrain. A low-magic world might have large warships outfitted with ballistae in the manner of traditional spelljammer ships, but at high levels the party can personally defeat such ships with ease, so they have no incentive to buy those kinds of ships or weapons. If they ally with a traditional navy to fight a larger force, have the mundane naval combat happen in the background as the casters rain ruin on anything in their vicinity and the melee fighters board the ships like marines and rip through their crew in a few combat rounds.
However, there are many alchemical, magical, or magitech items that can boost the capabilities of an adventuring ship or serve as on-board weapons.
Treat these as magic items. Whenever the party would receive a magic item from a treasure hoard, you can replace it with something that boosts the capabilities of the ship. Use the same rules and heuristics you would use for creating magic items and awarding them to the party. When in doubt, use an existing magic item and change its flavor. For example, instead of awarding a Necklace of Fireballs, you can give them a 'Goblin Rocket Pod' the size of a large crate that does the exact same thing when mounted on the ship.
Damage-dealing weapons that affect normal enemies using the normal combat rules are from the same tier, but armor or items for the ship are a tier higher. All people on the ship benefit from the protection, in addition to the ship. For example, a Cloak of Protection is an uncommon item, so basic ship shielding would be a rare item that gives a +1 to the AC and saving throws of the ship and everyone on it. A Cloak of Displacement is a rare item, so a displacement illusion engine for a ship that makes an illusion of the ship appear in a slightly different place would be a very rare item. Like a character, a ship can only attune to three items.
To randomly award such ship items, roll on the next lower table and adjust it to fit. For example, if a hoard calls for rolls on Table G, roll on Table F instead and make it a ship item. If it is a personal weapon, you can reroll, or make a larger ship-combat equivalent, or make it a point-defense weapon with unlimited uses. For example, if you roll a Wand of Web for a ship object, you could make it a device that can launch a large anti-ship or anti-asteroid web up to seven times and recharges when in port, or a magitech gizmo that lets someone sitting at the control panel cast normal webs at will to repel boarders.
If you feel like messing with ship weapons, a 'Weapon +1' table roll result could be a lightweight masterwork or magical ballista suitable for mounting on the bow of an adventuring ship. A 'Weapon +2' is a mangonel and a 'Weapon +3' is a cannon. In each case, loading, aiming, and firing can be done by one character with a minor and standard action, but only if the ship is pointed in the general direction of the target.
For an interesting combat encounter that leads to satisfying loot, roll up these kinds of ship items before combat for high-level ships that the party is attacking or boarding. Then after they win, they get to claim the items for their ship.
If you roll an item that gives a character a fly speed, make it a ship item that allows the ship to operate in gravity. This could be limited-time or difficult to use, like an inflatable gas bag that transforms the ship into a blimp or hot-air balloon.
Buying new ships and outfitting them makes an excellent money sink for the party, especially in a setting where it is hard to purchase magic items. If the party wants to buy or commission a ship item, have them roll a Persuasion check to represent negotiations. Success means that they find somebody that has or can make something of your choice. Higher results on the check mean more options are available; give them a menu of a few things to buy.
Most things can be handled with a simple abstract skill check. Set the DC based on what kind of campaign you want to run. I am running a fun friendly game where most things just need a DC 10 check to avoid problems, and high results can cover for someone else's low result. DCs will go up as they move into more hazardous and lucrative regions of space. I do not require all checks for all journeys; I mainly use them to add randomness and spice to the game. When appropriate for the story, I just have them arrive where they need to be without making any checks. For a more realistic and gritty game, require higher DCs for everything and/or have problems arise if anyone fails, which will require that the PCs build their characters around sailing or hire expert assistance.
Any character that is given or could learn the water vehicle proficiency instead learns the space vehicle proficiency.
A successful journey requires four checks: navigation, steering, piloting, and lookout
Navigation: Plotting a course is an Arcana check and/or an Int-navigator's tools check. Success means arriving on time, high rolls mean taking advantage of some temporary condition to get there faster, and low rolls mean losing time or possibly getting lost. The crew has disadvantage on this check unless someone on board has traveled the route before, or they have a good detailed recent chart describing the path. They have advantage if they have personally traveled the exact route in similar conditions a couple times before.
Steering: Ships are steered by hauling on the lines that connect to the wings and sails. This is a Str check for tactical movement, and a Con check for long-term operation, with vehicle proficiency added in each case. Having more people help lowers the DC. If the check fails, impose disadvantage on the Piloting check.
Piloting: Reading the weather and terrain and avoiding storms, floating space rocks, creatures, or magical phenomena is a Survival check and/or a Wis-vehicle proficiency check. Low rolls mean running into some hazard, possibly damaging the ship, while high rolls could mean saving time or finding something interesting to investigate.
Lookout: Being the lookout is a Perception check. Low rolls mean the party gets ambushed, and high rolls mean they get to ambush things or steer around them, or that they found something interesting to investigate.
Identifying ship designs and flags and knowing who is likely aboard a certain ship is a History check. Knowledge of naval and other military strategy is also a History check.
Unrepaired damaged ships lose range. Repairing the ship is an appropriate tool proficiency. Mundane ships and basic structural components are fixed with things like carpenter's and smith's and tinker's tools; sails are repaired with leatherworker's tools (sailcloth is as tough as leather); and magitech engines are repaired with things like glassblower's and jeweler's tools. Basic repairs use Str, Dex, or Con as the base stat the proficiency is added to. (Con checks are for tasks requiring long repetitive simple work.) Figuring out how to bodge together a complicated or extensive repair is a Wis check. The ship's range might also be upgraded with the same proficiencies, although that requires more raw materials, and coming up with the plans for the upgrade is an Int check with the relevant tool proficiency.
Alchemist's supplies can be used to make magitech fuel from raw ingredients, and calligrapher's or painter's supply proficiencies help make better maps and charts, or charts that can be sold to others.
Jumping and Moving in Zero Gravity
When jumping through zero gravity, the normal jump distances are ignored. Characters can go as far as they want, and can shoot things and cast spells while momentum carries them in a straight line. The difficulty is landing in the right place and not getting hurt when arriving at the destination. This is done with an Athletics or Acrobatics check. DCs to hit a target 5-foot cube increase with distance and relative velocity. Falling damage might apply if they failed the check or jumped into a gravity field or onto something with a high velocity. Failing the check by a lot might mean missing the target completely and drifting off into space. In one round, things will travel two to four times their movement distance, depending on the check.
Everyone without a fly speed will want to have at least one grappling hook on their person at all times. Using it to hook onto something is a dex-based ranged attack. Any character who has spent a lot of time in space and/or practiced with them can add their proficiency bonus to the attack. Moving along the rope (or any rope or climbable surface in zero gravity) can be done at the character's normal movement speed with no check if both hands are free.
There are also a variety of wildspace creatures that can serve as zero-gravity mounts. You can decide what they are, or let the players imagine what they want to ride. Use the cost and stats for a riding horse or warhorse with an exotic saddle, but with a flying speed. These creatures can only live in zero gravity and cannot move into gravity fields. (Mounts like pegasi that can fly in gravity are as expensive and/or difficult to obtain as they would be in a normal campaign). The mounts are stabled and carried along in pouches or nooks on the side of the ship. Mounts can often graze or forage depending on the wildspace terrain the party is traveling through, so carrying one or two will usually not decrease the range of a larger ship. If the mount is combat-trained like a warhorse, the melee characters can ride them in combat when the ship is attacked, using the standard mounted combat rules.
At higher levels, characters will want Brooms of Flying or Winged Boots for personal mobility in space. In all but the lowest-magic settings, these are standard equipment for space adventurers and most shipyards will sell them for 500-1000 gp. Riding a broom requires a free hand at all times unless the character has the Mounted Combatant feat. At your discretion, these items might only work in zero gravity, allowing for movement in space while keeping the party from having too much tactical advantage in dungeons and other ground adventures. You can also change the flavor, for example making them rocket boots or something like a magitech speeder bike.
Battle Grid Rules
Most ships will have at least a few locations where a person has three-quarters cover from most angles. Most enemies want to board the ship rather than trade shots at range, and will often take some ranged fire before they board. Increase the number and difficulty of enemies a bit to account for this, without awarding extra xp.
When things attack a moving ship, they have to spend 20 feet of their move keeping up with it. Damage to the ship lowers this number. If they want to move and attack, they can move their speed minus 20, but usually they will double-move to try to board. Most enemies will try to start behind cover or concealment so they can get close without being seen and board the ship quickly.
If the characters want to move the ship on the battle grid, the person at the helm makes a Wis or Int check, adding vehicle proficiency, and one or more other people makes a Str check, adding vehicle proficiency. Take the highest of the Str checks, and then the lowest of that number and the helm check. Divide by 5. That is the number of squares they move the ship on the grid, in any direction. The crew can make a 45-degree turn instead of moving five feet.
The ship can be damaged by a powerful attack. Enemies might damage it to make boarding easier. For my game, I have the first attack 'shake' the ship to warn the players that another one might damage it.
Use the rest variant where a short rest is a night's sleep and a long rest is several days of recovery. Require the party to be in a port for a long rest. While in port, the party needs to spend 10 gp for the lodging, meals, and amenities needed to benefit from a long rest. (Characters with background or training relating to wilderness survival might be able to make a campsite or base that counts as a port, if they are on an island with hospitable terrain.) This rule makes each 'adventuring day' a different voyage of the ship, and makes it very important to maintain access to friendly ports.
The level of encounters on an island, either natural monsters or sapient troops, is based on the ecological richness of the island, which can be estimated by looking at the island from a distance. A DC 10 Nature check gives the approximate level, and a DC 15 check gives the exact level. This information may also be on charts that can be purchased or found.
Spelljammer inherently is tied to the feel of the Age of Exploration. It is an excellent opportunity to explore ideas and settings relating to that time in history.
My game setting for the previous game was similar to 17th century North America. My knowledge of that region's ecology, culture, and history made it easier to describe things and invent details on the fly. Depending on your knowledge and interest, you could also do Mesoamerica, the Caribbean, the South Pacific, Africa, Asia, or any other place affected by the Age of Exploration.
Given their place in spelljammer lore, I suggest making the elves similar to the British empire. High elves are the government, navy, and aristocracy, and wood elves are colonists or pilgrims looking to make a new freer life in the 'wilderness'. They generally consider themselves loyal citizens, but that might change. In my games, the elves, like the British, have the most powerful navy and are consequently the dominant geopolitical force in wildspace. Their society is based around magical biotech, and they grow their powerful living ships from seeds in special locations.
It works well to have the other traditional D&D races of dwarves, halflings, and humans be like other Europeans, i.e. alien invaders from a faraway land. All of the other races can then be the natives of the land your game takes place in. In my game, they were similar to Native American cultures like the Iroquois confederation.
If you have players who do not know the setting, or just want to play a normal D&D character, or do not have a lot of time for preparation and world-building, let them use a traditional D&D race and be a soldier of fortune who joins a ship's crew or is hired to do something. They learn about this new world at the same time their character does. Players who want to make a rich backstory can describe their character's tribe and culture, and be a native of the land.
This all naturally leads to themes of exploration, imperialism, and colonialism with rich narrative potential and opportunities for character growth and conflict.
In my current game, I am using the historical approach less and instead adding in a lot of the fun stuff from Planescape and Eberron. It has much more of a sci-fi, noir, and pulp feel. Adventurers congregate in fueling depots on island outposts that have lots of different races all mixed together. The tavern in the player's home port is kind of like a cross between a dive bar in Sigil and the Mos Eisley cantina.