Divide teens into two groups. Give each group a water gun and a source of water (bucket, hose, etc). Pick two volunteers to start. Have volunteers stand 15 feet away from each other and tell them to use their water guns to shoot the shaving cream off their opponents head. once the pile is gone (or watered down sufficiently), give the winning team the point for shooting the pile off, have kids go to the end of the line and do the whole thing again. The team with the most point wins.
Thanks to Pastor2Youth.com for lending us this game!