Go to promiseofplace.org for a two page pdf of research summary collected by Louise Chawla on the benefits of nature for children.
These include:
Improving concentration and impulse control
Assisting children with handling emotional stress
Inspiring more creative play
Fostering environmental stewardship
Reducing ADD and ADHD
Improving motor skills
I have appreciated Dr. Chawla's work which has been referenced in Richard Louv's book Last Child in the Woods as well as David Sobel and other educators' work.