The kids in my youth group LOVE this game! Divide your group into 4 equal teams, and have them stand in 4 different sections around the playing area. In the center have a BIG bucket full of water & sponges.
Each team is given a chance to declare war on another team. The team that declares war runs to the bucket and gets a sponge and gets to throw it at the members of the team they declared war on, who can only defend themselves. The kids love to wet each other.
We originally played this game with water balloons, but you need a large supply, which takes ages to make! So now we use sponges. Try it out, the kids will love it! Just get them to bring a change of clothes with or towels so they don't wet their parent's car after wards!