How do you make dungeon rooms more interesting through various trappings & details?
First (inferior) answer: If the room is likely to be a combat location, or you want it to be exploited as a possible combat location, then add anything which might affect combat: Barriers, arrow-slits, trenches, a ballista, murder holes, crenellated ledges, substances that might hinder escape / pursuit, distractions to allow ambushes, etc.
Second (better) answer: Consider thinking concept-first instead of trappings-first.
Trappings don't automatically make rooms interesting. Interest is built by the presence of meaningful choices in the environment. Come up with the concept of those choices first, and then the trappings naturally suggest themselves.
For example, let's say you want one of the underlying concepts for your dungeon to be "a place where creatures (including PCs) can be chopped apart (sometimes voluntarily) and reassembled into amalgamations, with capabilities appropriate for the parts removed & added."
From there it's easy to come up with a variety of trappings that help communicate and enable meaningful choices:
- detached (and still animate) heads in jars of "life fluid"
- cold storage areas with all manner of limbs
- carts with straps for transporting "patients" around
- various saws mounted in the walls
- special guillotines of various sizes
- journals, books, or charts with relevant research / clues as to the purpose of the dungeon, and or its operation (heh)
- containers of flesh adhesives
- suturing gear
- recovery beds
- cages with captives destined for disassembly
- summoning areas & gear, to bring in "special" candidates for disassembly
- makeshift graveyard for unwanted remains of candidates