Turning 13-- BIG THING

I am turning 13 in October. I am having a big party in my back yard and it is my first girl-boy party. I am having 15-20 people coming. I need some Ideas of things to do!! I already desided to have a ton of contests-- like sack race, bobbing for apples, and even a pie eating conest!!! I need at least one more thing to do because it is going to last 4 hours. Turning 13 is really big for my family and school. Please help make mine the best!!!
Thanks,
Cecelia