I've been wanting to do an upside down room! Cemetery is the first one to come to mind. If there's enough height, take some brown cloth and spray paint patches like black and green. Staple them to the above railing / support frames and then hang tombstones up, upside down, making sure there's enough room for everyone (use small headstones) and buy /make a fake tree upside down in the corner. Now, put an actor in a green ghillie suit or make a suit to blend into the tree top (it'll be on the floor, remember the tree is upside down!)

Put a single small strobe or light facing the patrons before they enter. Not blinding them, just blurr their vision a bit. While they're looking up at all the tombstones, the actor in the tree top comes at them.

This is what I had in mind. I don't know how expensive it would be, black paint for walls, cheap foam tombstones and a fake tree. I think everyone should have a green suit for actors. If not, they should get one! always find a use for them.