Oozes and Elementals

My Lego collection from my childhood had a lot of spaceships. For a while, all of the cockpits and windshields sat in a box, because I could not think of anything to use them for in a fantasy world. Then I realized that they could be put to good use by making slimes and oozes:

Having things engulf the characters is an excellent way to make combat interesting. It is a lot of fun to actually put the minifig inside the ooze, and Lego cockpit pieces are perfect for this. The build is really simple: just attach the cockpit to whatever similar-colored bricks you have, and leave enough room in there for a minifig.

Air and water elementals also have attacks that engulf people. The same pieces can be used for oozes and water elementals, but air elementals should be a bit more loose. Instead of making an enclosed space, just pile up windshield pieces:

And of course, no game of D&D would be complete without a Gelatinous Cube:

This is an actual cube; five bricks high is the same distance as six pips wide. It is exactly 10'x10' on the dungeon tiles, the proper size for a Gelatinous Cube, and most minifigs will fit inside.

As you can see from these pictures, you do not need to spend a lot of time building clever or complicated things to be a Lego GM. Just throw together something that conveys the idea of what is happening, and let everyone's imagination handle the rest.