My son is REALLY into undersea stuff so it was no surprise that he wanted an undersea party. We reserved one of the shelters at a nearby playground and hung streamers and fish decorations from the ceiling. He loved them so much he hung them in his room later so it was like an aquarium!  I made the cake in the shape of a fish by cutting the corners off a rectangle cake and putting one triangle on top and bottom for fins and the other two on the back for a tail.  He was so surprised!  When his friends arrived we had a table set up with crayons and paper with fishy borders to color that we found in a pad of paper from Crayola. Once everyone arrived we played a sharks and minnows type running game. When everyone got tired out we played the "undersea action game".  It was a Simon Sez sort of game that my son and I made by cutting out fish from construction paper (if I did it again I would use that thin foam instead because he ended up saving the fish to play with later) and writing actions on them like "Make a train with your friends", and "Pretend you are swimming", "Jump up and down" etc. I attached paper clips to each one and my son decorated the other side with markers.  We put them decorated side up on the ground and the kids all got a turn to "fish" with a pole I made with a stick, string and magnet. They all had to act out whatever it said on the fish they caught.  I got some great pictures during this game! After cake and pudding cups (ice cream would have melted) the kids played another fishing game.  I replaced the magnet on the fishing pole with a clothes pin and hid behind a part of the playground with a basket of "beanie fish" that I got onsale. Everyone reeled in whatever I clipped onto their line. This was their take home gift. Before everyone left I got a picture of them all holding up the "catch of the day".  They played on the playground for the rest of the time and invented several bean bag games of their own. It was a great party...maybe too good...how can I top it this year????? :)