If you've got a Wii, gather a group of friends and challenge them to a competition using Wii Sports, (this game comes with the Wii)! You can play tennis, bowl, play baseball, and even box each other! If you've got some of the other sports games on the Wii, mix it up!
If you're feeling creative, stop at the party store ahead of time and pick up some cheap medals. (Or make some out of ribbon and construction paper.)