Look under your chair and what do you see? A diaper with fake poopy? You win a prize!
Diapers and a prize
Get a package of baby diapers for newborns and put one under each guests chair. In one of the diapers paint a big brown splotch (baby poop!) or put some playdoh etc. Be creative! During the party when all the guests are seated tell them to look under their chairs, and the person who gets the "surprise" gets a prize! Give all the diapers to the person being honored.