I stumbled across this and just had to share:
http://www.steve-olson.com/child-creativity-linked-with-outdoor-free-time/
It's a playground at the University of Minnesota Arboretum and it looks like a ton of fun. It's all natural -- sticks, canvas, tree stumps -- and invites a lot of imaginary play. What's really nice about this web page is that he included a bunch of photos of how his son interacted with the play areas. Somehow, seeing the kid jump and play makes a line of tree stumps make more sense as a play area.
A lot of it is really simple. There might be ideas here that you can incorporate into your projects.