The Social Life of Small Urban Spaces (with vintage Adventure Playgrounds), William Whyte, NYC, 1980

William H. Whyte - Social Life of Small Urban Places from Robin van Emden on Vimeo.

William H. Whyte's film "The Social Life of Small Urban Spaces" is an urban planning classic.  Whyte moves through the mid-century plazas of New York City in a direct, shoe leather observation of why some spaces work, and some don't; complete with old school charts and graphs.
     
You really should watch the whole thing (it's 58 minutes)--the architecture and people are a delight--but pay particular attention to the footage of Harlem street play and kids in an adventure playground, at 1:30 - 2:40, and Whyte's surprising observation that the playground space was actually under, not over, utilized.

Every designer of outdoor space should be required to watch Whyte's discussion of the characteristics of a good bench, and many of his observations apply to playgrounds just as well as plazas:

"The number one activity is people looking at other people, but it is a point that is overlooked in many many designs"

"Visual enjoyment (of a site from the street), this secondary use, is every bit as important as the primary use." 

"The 'corner' is a sociable space."

"People don't like to talk in the middle of a large space.  They like to find places like steps, edges, flagpoles."

"The places that people like best are those which open to the action but are slightly recessed, slightly protected. You get a caving feeling.  Just a few honey locusts overhead will do it." 

"To make a place like this work, you must unfence it!"

Read more about William Whyte at the Project for Public Spaces