My personal favorites....
10. Move your roommate's personal effects around. Start subtly. Gradually work up to big things, and eventually glue everything s/he owns to the ceiling.
13. Spend all your money on Transformers. Play with them at night. If your roommate says anything, tell him/her with a straight face, "They're more than meets the eye."
27. Hide a bunch of potato chips and Ho Hos in the bottom of a trash can. When you get hungry, root around in the trash. Find the food, and eat it. If your roommate empties the trash before you get hungry, demand that s/he reimburse you.
44. Shelve all your books with the spines facing the wall. Complain loudly that you can never find the book that you want.
68. Whenever the phone rings, get up and answer the door.
69. Whenever someone knocks, answer the phone.
78. Take all of your roommate's furniture and build a fort. Guard the fort for an entire weekend.
92. If you know that s/he is in the room, come barging in out of breath. Ask if they saw a fat bald naked Tibetan man run through carrying a hundred dollar bill. Run back out swearing.
98. Hang stuffed animals with nooses from your ceiling. Whenever you walk by them mutter, "You shouldn't have done that to me."
110. Become your own twin brother and tell your roommate that you and your brother never appear in the same place at once. Tell your roommate the same thing again after leaving and coming back into the room.
157. Eat lots of "Lucky Charms." Pick out all the yellow moons and stockpile them in the closet. If your roommate inquires, explain that visitors are coming, but you can't say anything more, or you'll have to face the consequences.
183. Wear a cape. Stand in front of an open window for about an hour every day. Then, one day, when your roommate is gone, go outside and lie down underneath the window, pretending to be hurt, and wait for your roommate to return. The next day, start standing in front of the window again.
184. Collect potatoes. Paint faces on them and give them names. Name one after your roommate. Separate your roommate's potato from the others. Wait a few days, and then bake your roommate's potato and eat it. Explain to your roommate, "He [/She] just didn't belong."
201. Scatter stuffed animals around the room. Put party hats on them. Play loud music. When your roommate walks in, turn off the music, take off the party hats, put away the stuffed animals, and say, "Well, it was fun while it lasted.
248. Create an army of animal crackers. Put them through basic training. Set up little checkpoints around the room. Tell your roommate that the camel spotted him/her in a restricted area and said not to do it again. Ask your roommate to apologize to the camel.
317. Arrange your pillows and blankets every night to make it look like you are asleep. Do this for three weeks. Buy a cantaloupe and a knife. Stick the knife in the cantaloupe. Lay it on the pillow where your head should be.
355. Make your bed 15 times a day. Sleep on the floor.
Complete list here...