If your birthday boy or girl has a particular penchant for treasure hunting, pirates or adventures on the high seas, why not plan a birthday party around that? You could pick up a Jolly Roger flag from a party supply store, or make your own with an old pillowcase. A pack of costume eye patches and temporary tattoos will instantly turn guests into a roving gang of pirates; then, as a craft, have kids make and decorate pirate hats.
Of course, you can't have a pirate or treasure-themed party without a proper treasure hunt. Either draw out a map to the booty or set it up like a scavenger hunt with clues at each stop. Be sure to have plenty of gold chip candies for the prize and some inexpensive plastic costume jewelry for girls. A simple game of musical chairs can send players walking the plank if they don't grab a seat in time, or a three-legged race could be converted into a peg leg dash.