Prank ideas?????????

okay so at my party my best friend had everyone say they wanted a group picture then he pied me in the face.... it was great! i gave him the congratulations he deserved! but now its my turn to get him back! see he is having a party in about a month and i really want to get him back! i was thinking somewhere along the lines of water balloons, silly string, and a couple pies to the face.... any good ideas out there??