partially fill a paper bag with flour and stick it under someones door and jump on it or drop a heavy book on it. it covers the whole room in a fine powder
Take some double sided tape, some bouncy balls, pennies, and 2-liter bottle caps (or anything else that is small and unnoticeable) and tape them to a ceiling fan.
WHAM! BOING! PING! CRASH!
I prefer this one, since it's not really cruel, and any injuries inflicted usually heal within 6 months.