A wet-n-wild water party.  My 3 year old daughter loves taking baths and playing in water.  She is also a big fan of SpongeBob Square Pants.  We bought a small kiddie pool and a slip-n-slide where the kids run and slide on their bellies.  We filled the kiddie pool with plastic sea horses, fish, wind-up whales, turtles and frogs. We tied balloons to the arches of the slip-n-slide that had the number 2 on it and we also tied balloons around the backyard and on our deck. We also hung a pinata (can buy shapes of fish, sea horse, starfish, etc.) in the yard and filled it with candy for the kids to burst. We decorated cake with the icing in colors that looked like the bottom of the ocean and used a candle that was shaped like a 2.  We had the classic pin-the-tail on the donkey only instead of a donkey we copied a fish onto 11x17 paper and made little tails with small pieces of tape on it to stick to the 11x17 paper.  I printed invitations on pre-printed paper with fish and sea life on it.  I wrote on each invitation to expect to get wet, to bring a bathing suit or dress in something they wouldn't mind getting wet as well as to bring their own sun tan lotion.  Everyone enjoyed the party and several months later I spoke to a 5 year old who had attended and she asked when I was going to have another water party. She said she had lots of fun.  So I know it was a big hit. Additionally, the water and sun wore out the smaller children and they slept very soundly that night from what some parents told me.