Researchers at the University of Illinois at Urbana-Champaign concluded that children with ADD showed a dramatic decrease in symptoms when playing out in nature (green areas) and/or playing in rooms with windows. Sited in their research were several other studies that concluded green environments around schools and window views benefit all children by fostering creative play.
Taylor, Andrea F., Frances E. Kuo and William C. Sullivan 2001. Coping with ADD: The surprising connection to green play settings. Environment and Behavior, vol. 33, no 1 (January), pp.54-77.