An excellent presentation by Stuart Brown about the role of play.
Here are some of his quotes to get you motivated to watch it:

“If the purpose is more important than the act of doing it, it’s probably not play.”
“Nothing lights up the brain like play.”
“The opposite of play is not work. It’s depression.”
“The thing that’s so unique about our species is that we’re really designed to play through our whole lifetime.”
“Play has a biological place. Just like sleep and dreams do.”