For a maybe less cruel toilet-related one, put some kinda large fake bug somewhere that someone is likely to only see it when they go to the bathroom. That happened to me once with a non-prank real centipede, about 2.5" long. It was a really big one, biggest I've ever seen. Looked up just as the urine started to flow... that stung a lot! I let the thing live, mainly because I didn't want to go anywhere near it, and especially didn't want to clean up the mess if I threw something at it, or shot it with an airsoft, or something!