There are some great articles in latest issue of Greater Good Magazine on the importance of play, with some great advice for parents.
This quote from David Elkind:
"Over the last two decades alone, children have lost eight hours of free, unstructured, and spontaneous play a week. More than 30,000 schools in the United States have eliminated recess to make more time for academics. From 1997 to 2003, children's time spent outdoors fell 50 percent, according to a study by Sandra Hofferth at the University of Maryland. ...Decades of research has shown that play is crucial to physical, intellectual, and social-emotional development at all ages. This is especially true of the purest form of play: the unstructured, self-motivated, imaginative, independent kind, where children initiate their own games and even invent their own rules."