From Green Hearts Institute for Nature in Childhood is located in Omaha, Nebraska. They've put out a nice document of best practices when considering and designing nature-based play spaces. There are some interesting sections on risk, prospect and refuge, water, loose parts, and some good advice about what might and might not fly in terms of liability.
Design Principles from Green Hearts
TrackBack URL: http://blog.lib.umn.edu/cgi-bin/mt-tb.cgi/152772