Last year we had a "Wet and Wild" party for my daughter's sixth birthday. We don't have access to a neighborhood pool, so we did the next best thing.
We made the backyard into an obstacle course of water activities. First, we took our Little Tykes slide and fastened our water hose to it. The kids enjoyed going down the water slide and ending up on a plastic liner we had purchased at Home Depot (after the party we used the liner to cover up firewood in the backyard). We had a slip and slide, a child's pool, and a sprinkler set up with the "Crazy Daisy". After playing on the obstacle course, we settled down and played water games. The first game was a version of duck, duck, goose, but when it came time to "goose" the person, the player poured a glass of water over their head then ran around the circle to their seat. We played water balloons and then had a Mermaid Piņata. Each child's goody bag was a bucket with a beach shovel attached to it filled with a pair of sunglasses, visor, and a beach ball, all of which were purchased at the dollar store. Finally, my husband took individual pictures of each kid along with a group shot of them in the pool. When we sent out our thank you cards, we inserted their individual picture and group shot picture in it. The party was a big hit! In fact, many of the parents ended up staying at the party because of all of the excitement.