What are some things adults can do on a 3 day weekend?

I'm coordinating a 3 day weekend for my frat bros and our wives in July. The first night when everybody gets here we're going to have a cookout here at my house, but that's all I can think of to do. I live in L.A. and the only other thing I can think of that everybody might like is going to the beach. It will be about 15 of us, and our ages range from about 25-33. Any ideas?