a bed, table, chair, and fireplace,
terrain pieces with hidden compartments,
a ladder down to a lower dungeon level, a shelf full of magical items,
a brazier, a throne,
a feast table, a rack of kegs,
a council table,
or just a basic a table and chairs.
Individually, these little builds may seem boring or simple. But when you have tubs full of them,
you can make entire dungeons full of fun and interesting encounters in a hurry.
I often find that the best additions to my dungeon dressing tub are the little terrain pieces with play features from the lower-cost Lego sets. For example, Fire Dragon's Lava Cave has both a sliding-lava-waterfall terrain piece and a terrain piece that transforms a pool from green water to clear water. I removed the latter from its green base and made a dungeon dressing piece that can be put anywhere.
This treasure chest statue on top of a chain-activated trapdoor comes from the Oasis Ambush set from the old Adventurer's line.
Many modern sets have excellent little terrain pieces with well-designed play features. Use them for inspiration, or download the instructions and make copies of them from the pieces in your collection, or just buy them, build them, and keep them in a tub to use for your games.