My husband (a big kid at heart) had his 22nd b-day when we were only dating.  I worked at a child development center so I decided to hold him a party that any kid would love! I decided to have a Birthday party with an "Under The Sea" theme. We had Fish Piñata (made from a huge balloon, paper mache and tissue paper), green streamers from the floor twisted in various lengths up the wall, tables and middle of the floor for seaweed, blue streamer and 3 shades of blue balloons hanging from the ceiling for the top of the water and bubbles, and various other props. We chose one corner of the kitchen to do this in. We made our own cake, but you could also find a fabulous bakery to make up a good fishy cake. Use anything with the beach, fish, or the underwater for inspiration.  Too cute.  With the CD's out there now, you shouldn't have a problem finding ocean sounds. The piñata was full of candy and prizes easily found at a party goods store. There were a few grownup prizes there to. Our nieces and nephews loved the piñata.