Put someones phone/money/keys in their shoe, fill their shoe with water and then place it in the freezer.
Glad wrap over the toilet.
Place peebles in ones fuel tank.
Create a .bat file that deletes system32 and then run it on their computer.
Have sex with their cat and post photos of you having sex with it on facebook like 'Yo, this is your cat'.
Place commercial quantities of drugs in their house/unit/apartment and then notify the police that you suspect a big time dealer living in that area.
Park their car in their garage, then light'em'up until the tires go. Make sure the garage door is shut but any internal doors leading to the house are open.