I realized that one of the senses I neglected was that of motion. Here are some ideas it might be cool to experience in the invisible:
- a trapeze
- a rowboat on invisible water
- swiveling or rotating platforms (think gears)
- draperies hanging from a ceiling
- breezes & drafts
A long table set with strange and alien fruits . . . turns out to be some sort of banquet. Hopefully this will intrigue players "Why is this food still fresh? Who was this table set for?" and play with their expectations-- "This fruit is a grape-like cluster of what feel like peppers. Tastes like . . ."
Watch out! broken glassware on the floor, pungent fluids spilled . . . must have been some kind of laboratory. How do you safely walk through here? Is this gunk on our feet leaving tracks? Is there anything in here that could follow them if it is? What can you find that may be of value? Unbroken glassware? Stoppered phials partially filled with liquid?
Shelves full of books and scrolls . . . a library. But which ones do you take? Is this gold embossing? This one has a bunch of dog ears, must have been useful to someone. Or do you just go for the biggest, or smallest? (I remember fondly spending time in my university library's section of over/undersized books. What were they fff, uu?)
And how about rooms/environments so puzzling that even you're not sure what they were meant for:
A room of evenly spaced, waist-high, granite posts about a hand's breadth wide. Imagine trying to fight amongst these, or flee through them. But the adventurous soul that walks along top of them finds certain posts taller than the others, that lead to other even taller posts-- a sort of hidden stair.
A chamber filled with shallow troughs of liquid in patterns on the floor and large stone spheres nestled in them. Rolling the balls in various directions in the troughs diverts the liquid in different ways, revealing a passage, a chest, a . . .? Or maybe they do something somewhere else in the dungeon?
What would intrigue you to find in an invisible room?