Bank - lots of money dollar bills, gold coins, silver coins, bronze coins. The exit is a bank vault, and can also serve as a locked exit - you just need to collect all the keys.
Well look at that - pretty much exactly what I have just created, without having looked into this thread before
. Just like instruments.
Math, which has been mentioned here, too, is also something I have considered - though it would probably contain a lot of geometrical shapes, too (triangles, polygons, mathematical operators, etc.)
I have also been toying with the idea of a medical tileset - containing syringes, scalpels, stethoscopes, as well as a bunch of human organs. Especially the latter will be dificult to sprite, though.
I do have a lot of potential level titles for such a graphic set, though: "Lemergency room", "House, Lem. D.", "Let's get physical"...