Water-Balloon Party - I had this party for my 9 year old son's birthday this year in July and the kids had a BLAST!!
Make up as many water balloons as you can. I think I had about 20 kids and 300 balloons. There are never enough! I divided the kids in to two teams as evenly by age as possible. Then we played games like relay races, trivia questions, musical chairs, and a scavenger hunt to WIN balloons for each team. The winners of a game would get 30 balloons and the losers would get 15. Then when the games were over, I put a hose down the center of the yard on the ground so each team would have a "side" and the rule was they couldn't cross over the line. This eliminated "ganging up" and nobody got hurt. Then they all had a water balloon fight. A better idea might be to put a sprinkler on in the middle so everyone was getting sprayed too. After the water balloon fight I gave everyone a party bag of candy AFTER they traded me two handfuls of broken balloon pieces.