I was that guy in college.
Started a 'war' against my neighbors in the dorm. Learned to pick locks to get into their room whenever I wanted.
-Filled room with popcorn
-Filled room with newspapers
-'flipped' layout of the room including the keyboards
-covered doors of the entire dorm floor in ducktape
-replaced pillows with water balloons (and some needles to pop them when touched)
Things they did back (to me and/or roommate)
-wrapped us in plastic wrap
-switched the harddrives on our computers
-baked us a giant bread "male part"
-mailed us a gallon of milk, took 2 weeks to get to us.
-glitter bombs
-completely changed our door lock
Next year, I move into a 20-guy frat house and started another war against everyone (although people didn't know who the mastermind was). A lot of this was with help from other guys, but it was usually my idea.
-regularly signed people up for tampon samples and diapers (Again, all guys)
-sent people x-small condoms, just cuz
-magazine subscriptions to teen weekly and other such wonderful publications
-Got a group of 10 guys to build a brick wall in the middle of someone's room
-switching shower gel for bbq sauce/ketchup/mustard
-installed a walkie talkie in the AC vent in someone's room, gave him 'ideas' for 3-4 days before he almost went crazy
-completely moved a guy from his room to the basketball court while he was using the bathroom (everything was tossed out his window and setup outside)
After that, I lived by myself for my own safety. No one knew it was me in the frat house until the day before I graduated when a group of us got together and someone brought a box of all the diapers, tampons, magazines, etc he was sent over the years and I just burst out laughing. I heard it's now a tradition for whoever get's my old room in the house to be sent tons of random stupid crap (something I still do when I'm bored). So it's one of the nicer rooms, but you have to deal with all the mail crap.