I work in an office where harmless pranks are not only allowed, they are encouraged! For my office managers birthday, the company president gave me the green light to decorate his office, top to bottom! (All black "over the hill" stuff of course) and the other guys are always doing stuff like putting all the unused chairs into the victims office when hes at lunch.
What pranks have you pulled?
(this is what I did when the office manager took the day off a week before Christmas)