As San Francisco's largest and most relaxed beach, Ocean Beach is a delightful break from urbanity. Even on the warmest days, there is plenty of room to throw down a towel (or t-shirt) and soak up some California rays and play in the surf. It's advisable to bring wind protection and a warm layer, just in case :)
At night, bonfires light up the pits - for those arriving after they're (especially on weekends) a friendly approach (and some firewood/beverages/etc to share) should help make some new friends. Again, bring warm clothing! The first Saturday after New Years, Ocean Beach plays host to the slightly less than legal Yule Tide Pyre of Christmas trees.
The Cliff House and Beach Chalet provide indoor eating with expansive ocean views - Park Chalet (on the backside of Beach Chalet) serves outdoor cocktails on warmer days :)
Website: http://www.parksconservancy.org/visit/park.asp?park=68
Wikipedia: http://en.wikipedia.org/wiki/Ocean_Beach,_San_Francisco,_California
