When my oldest son turned 5, we had an 'under the sea' party.  About three weeks before, we made a huge shark out of newspaper, stuffed him with more newspaper, and painted him.  We did the same for a big jellyfish. We also learned about a lot of different fish that would be found in a coral reef and made them out of construction paper. These decorations were all hung from the ceiling of our dining room. I got a couple of rolls of the blue Saran Wrap and we put it over the windows and doorways to make the whole room an ocean. It was fantastic!  Of course, we played the fishing game (my husband pretended to be 'the old man from the sea' and helped them fish). Their prizes were little plastic sea creatures, which they all loved. We also played ocean trivia, that was a hit as well. All of the treats (ocean stickers, shark gummy treats, etc.) were put into brown lunch bags that we glued 'teeth' on the top (white construction paper) and wiggly eyes on the sides.  I made a hammerhead shark cake.  The kids had a blast and we learned a ton preparing for this very inexpensive party!