ok, change windows startup sounds to **** soundtracks with moaning, etc.
change their door locks, so they cant get it
blow up condoms and add some spit\other substance that looks like jizz, put in bed so when they jump on \get in, it breaks and gets all over
unscrew and take out the light switches when they are out for the day.
take his room mates tampons \ pads, and put them all over their rooms, wet their underwear and put it back
shit in the top portion of the toilet, already mentioned
white out on the sheets and pillows, they get in and BAM they're white
ABC gum in leftovers in the frigde.
Take EVERYTHING from their room, and put in a storage facility, they think they were robbed.
buy a LOT of paper, fill their rooms so when they open the door, it floods out.
hide a speaker somewhere in their room and playback ****\farts\other offensive material.
some of the ones I've done, seen, or have been the victim of...