I work with a guy that always thinks it is hysterical to run around with an empty stapler. He'll go up to a female and say, "Hey, watch this!" and then he smacks his arm with an empty stapler. The girls all squeal and get grossed out. So one time, right after doing his stapler trick he turned around to answer the phone.......we put staples in.
I'm in the military, and for the first 5 years or so I was always around dorms. Young enlisted guys do some pretty stupid stuff, and it was common--no, it was EXPECTED to Tango Bravo somebody that passed out early. Is it pleasant? No,... but you'll never fall asleep around a bunch of soldiers again. And the memories last a life time rofl.
A kid I went to school with would do a #2 into a ziplock baggie and squeeze all the air out. Then he'd lay it in the middle of a street and watch a car run over it. You just know that driver won't even think twice until he gets home and gets out...
If somebody leaves their computer logged on, we'll run over and change the video settings. Most pcs give you the option to invert the screen, or set it sideways. When they go back you'll see them screwing around forever trying to fix it.
Put a piece of tape under their laser mouse. It still works, but very poorly.
Go into settings, you can reverse the mouse buttons, so a left click functions like a right-click and vice-versa. Also, while you are there sloooooow their cursor speed down. When they move the mouse the cursor will barely move.
If they use MS word a lot, you can go in and change the autocorrect options so that it will automatically change a normal word into something else. So everytime they type "the" it will instantly autocorrect to "teh". Plenty of irritating things to do.