The Thief

Take/steal your friends or roommates most necessary items. The little things that they take for granted are the best. Deodorant, chapstick, ID card, a belt, anything of this nature would work. If they ask you if you took it then deny deny deny...it's not lying, it's just a prank. Strategically place the item some place in due time...or never give it back just to piss them off even more.