The Nature of Outdoor Play is important:
Michigan Television Link: "Where Do Children Play" : http://www.michigantelevision.org/childrenplay/
This is a great resource on information on outdoor play. The various pages have good links to outdoor ideas as well as an introduction to a great documentary on the study of children's play.
"Go See It" Ideas: Nature Hike at the University of Michigan: Matthaei Botanical Gardens and Arboretum on Dixboro Road in Ann Arbor.
http://www.lsa.umich.edu/mbg/see/MBGDiscoveryTrail.asp