Items Needed: large tarp, a few packages of shortening (like Crisco), jello that has been pre-made (in a few large buckets), 1 watermelon per match.
Setup the tarp and secure it to the ground on grass, have leaders rub the shortening all on the tarp. (Warning: make sure no one stands on the tarp any time as it is extremely slippery). Throw on the jello and rub shortening on the watermelon.
Gameplay: Have two teams start at opposite ends of the tarp (around 5-10 players a team) on their hands and knees. Once go is called, teams pass the watermelon around to score by passing it past the end of the opposing teams tarp. This game is extremely messy. You can also use a hose for extra fun!
Bathing suits and messy clothes are definitely needed.