One side project I've been doing slowly is to create a platform where Melissa and I can practice martial arts. I put this down in the valley to the north of our house spot (it's about a 200 yard walk down to it from the bathroom).
On top of the redwood beams, I put a lot of different (scrounged) 2x4s, then two layers of half inch plywood. it ended up being 20x20. Now I'm going to start looking for some sort of padding... carpet, or astroturf or something. Someday I'll build a fancy roof for the thing.