It is a standard message from the paleo-crowd that 'play' should form a pillar of your life. This is quite a profound message given the physical benefits of vigorous play, the mental benefits from the fun of play and the wider skill development it implies.
Viewed against the barren and unstimulating environment of the modern gym (hey, have you seen the number of screens and mirrors they put in gyms to distract you from boredom and to lure you in to narcissistic motivation), play has full paleo credentials.
I found an interesting film on TED that discusses the importance of play. It is well worth 25' 42" of your life!