Here are five other was to expose less-than-classy guests:
1. Keep an open envelope out with a bill or statement inside. On the back rub graphite from a pencil. It'll stick on their fingers.
2. If ping pong balls aren't at your disposal, try filling the bathroom cabinet with something noisy.
-Silverware
-Nuts and bolts
-A pot lid
3. Put a note in your bookcase that says a personal, "Hello, _____!" When they look over your shelves, they will see your greeting.
4. Just before they leave, tell them you love to stay up after people leave, and watch the tape that the hidden camera takes. And check out the look on their face. (Maybe tell them you're kidding, so you can keep your friends/guests.)
5. Just after they use your bathroom, tell them a crazy story about the last unfortunate soul who looked through your medicine cabinet.
If you try any of these, tell me how it turns out!