So far I think the best has been when Will left his door open and we moved everything out of his room - clothes, laptop, cd, desk, drawers, wardrobe and mattress. Everything that wasn't screwed or fastened to the wall - which is only the bed frame. He went mental.
Plus we have a thing with the girls in E whereby we turn up drunk at any time of the night/morning and rob stuff from them. There has of course been the taking labels of cans, sellotaping up of foods etc.