Muir Beach is 30 minutes north of San Francisco. The beach is only 1 mile long, but it is a pretty, secluded spot. The locals surf here, but remember your wetsuit. The water is cold. A distinguishing factor of Muir Beach is that campfires are permissible. The last time I was there, the air was fairly cool and I was very grateful for another beachgoer's fire.
There is supposedly a clothing-optional (CO) portion of the beach, but I've never seen it.

Muir Beach is also the name of the community that resides near the beach.