Straight-up man-made dungeons are pretty boring until you start decorating...the list of things to decorate with are innumerable. Just to spice up the floor you could use some rugs, stairs, platforms, holes, pits traps, cracks, rubble, arcane circles, stains, and some light colored shading in the middle to denote "worn" areas where the people walk.

Once you start thinking vertically you start adding columns, bookcases, tables, desks, chairs, stands, musical apparatus, torches, beds, junk piles, coffins, chests, and all of the dollhouse amenities that you can think of...just leave enough room for people to move around in.