What are fun activities to do for an outdoor b-day party?

My daughter will be turning 11 this year and again, we're planning to meet at the park (with a playground) for her b-day party. It will be low budget because I just don't have the funds. We're going to have cake, munchies and water balloon fights. What else would be fun? I was thinking of having it from 1 PM to 3:30 PM. Is that long enough?