Home to two major museums - the Cal Academy of Science (which publishes a free app/guide specifically to the park) and de Young museums - a Japanese tea garden, a flower conservatory, a herd of bison, numerous lakes, a polo stadium, an early 1900's carousel, windmills, a Sunday roller disco, massive festivals and countless characters visiting on a daily basis - Golden Gate Park surprises with regularity!
Much of the park is blocked to automotive vehicles on Sunday making it a great time to explore. On the weekends - unless it's cold or raining - you can usually pick up a game of volleyball at Peacock Meadow, barbecue at Speedway Meadow, or rollerderby with the crew off JFK.
Bicycles (or other self-powered rolling vehicles) and feet are the preferred modes of park transport - for those seeking a ride, shuttle service runs on weekends and holidays.
Wikipedia: http://en.wikipedia.org/wiki/Golden_Gate_Park