Here are two more games we've played. Both are great, and both were from Familyfun.com, so go on over there for more ideas.
1. Fill the cup: This is a partner game.
The first partner gets a water squirter and a supply of water to reload if necessary.
The other partner gets a cup and stands across from the squirter. The object of the game is to be the first pair to fill up the cup. To make it fair, we had the kids switch spots and have a turn doing both roles. This was a huge hit. It was fun to watch them try to figure out the amount of pressure they needed to get to their partners and the faces of the cup-holders as they got drenched by their buddy's poor aim!
2. Leaky Cup Relay: This is a good teams game.
In this game, each team got an ice cream bucket full of water, and a Styrofoam cup that had lots of holes all over it. On the other side of the yard, each team had an empty bucket waiting for them.