Here is an oldie but goodie that I try to use every April Fool's Day...
Some kitchen sinks have them spray hose thingies right next to the faucet. I don't know the technical term for them, but you use them to wash dishes and such (duh, of course you do, it's part of the kitchen sink). What I do is, get a rubber band and put it around the "trigger" of the spray-hose-thingy. Point it at the front of the sink where someone might be standing. Then, when they turn to water on, BAM they get sprayed!
Warning: DO NOT ATTEMPT IF YOU HAVE GREMLINS RUNNING AROUND YOUR HOUSE!!!