MOst of my pranking came during dorm days at college. It was pretty communal, we all left our doors open, so these required inside access. But here were my three favorites, if they are applicable:

1. Open can of tuna, add milk, place inside A/C vent. This is kind of a time-release prank... give it a few days, and boy does it stink. Because it gets blown throughout the room, it's hard to pin down the source.

2. Open a window, pour trails of corn flakes from the window ledge leading into the room and all over floor and the bed of the mark. This one was perfect timing- he brought a girl over for the first time to find a dozen pigeons had taken over the room, feasting on the cereal.

3. Bike lock around the neck, "lose" the key (the U-locks, will fit around skinny guys' necks, but will not slip OFF of ANYONE's head). If you can't get them to do it to themselves, as I did, (a challenge- ie "NO ONE, not even YOU, can fit THIS sucker around...), then just put it on them while they sleep. It's not debilitating enough to have to miss classes or tests, but pretty humiliating, because it is too bulky to be hidden with clothes. They generally wait a LONG time before going to the fire department (at which point you "find" the key).