For my son's ninth birthday we had a mystery/detective party.  The boys came dressed up as detectives and very eager to solve the mystery. They were surprised to see "police tape" (made from yellow streamers and the words "Police do not cross" written with black marker) We explained to them that "someone had come and taken the party bags. They needed to go around the house and see if they left any clues.  The boys found things like a ladder at the window, notes, footsteps, hat, pen, golf club, all these clues led them to our neighbor. (all was prearranged) The boys solved their mystery and we carried on with the party. The boys talked about this for months and I got in trouble from other moms for having such a fun party. It is one we will always remember!!!!