Musical sprinklers

Materials needed:

• A rotating lawn sprinkler hooked up to a hose
• Someone manning the faucet who is prepared to sing and provide the "music."
  

HOW TO PLAY: Start with the sprinkler turned off. Players must move around the sprinkler area, jumping, dancing or striking funny poses. When the sprinkler is turned on, they must freeze in position and get drenched until the sprinkler is turned off again.

More cool ideas, like a water-filled piñata, or pinaqua, made from a trash bag, here.
Water sprinklers on eBay
Sprinkler pictures on flickr