This article from Current Problems in Pediatric and Adolescent Health Care by Leyla E. McCurdy MPhil, Kate Winterbottom MPH, Suril Mehta MPH, and Advisory Committee Chair James Roberts MD, MPH reviews the evidence of the mental and physical health benefits associated with unstructured, outdoor activities and time spent in a natural environment. 

"Children spend more time indoors using electronic media and less time engaged in outdoor unstructured play...Pediatric health care providers should recommend outdoor activities for children and refer families to safe and easily accessible outdoor areas. Pediatric health care providers can incorporate this simple, lifestyle-based intervention into anticipatory guidance."