Vaseline is the best thing to put on toilet seats. Especially if they've been out drinking. Just imagine a girl, half drunk, trying to scrub essentially grease off her ass with her panties around her ankles. It's a sight for sore eyes.
Frozen shrimp works better than fish for almost everything. Put it in the toes of shoes, in ash trays in cars, in underwear drawers, kitchen cupboards, jacket pockets, slightly buried in houseplants... speaking of houseplants, water them. With bleach.
Unplug the fridge. And the clothes dryer. Reverse the hot and cold hoses to the washing machine, then turn off the cold - everything will get washed on pure hot. Welcome to a bunch of wool sweaters that are about 6 sizes to small.
Fill condoms with eggwhite by the dozen, and leave them in the living room couch. In the same vein, pick up some REALLY embarrassing sex toys and leave them where it will be assumed they are being used by the room mates.
Don't forget the potato in the tailpipe stunt.