I've been reading about the terrific necessity of free play, and I think we do pretty well with that in our house. It's called "ignoring the kids." Then they create war zones with their Nerf guns or snake train tracks around rolls or wrapping paper or make up carnival games where they throw stuffed animals at landmarks around the house for points (10 points for the windchime!)
I just caught them slapping their bellies in the bath in some sort of 2 man drum circle. I really couldn't orchestrate them better if I tried.