My daughter's birthday is in August, so I also look for something that will keep us all cool.  Last year, she was too young for a swimming party, so instead we had a splash time party.  I bought a bunch of squirt toys and water toys, a wading pool and one of those sprinklers that wiggle all over the place. I also made sure to put on the invitation that the guests needed to bring a swimsuit or extra clothes. On the day of the party, I set everything up in the backyard (of my mom's house, since we live in an apartment), filled all the squirt toys, etc. As the guests arrived, they changed into their suits if they weren't already wearing them, then went out to the backyard and had a ball.