Burning Man's original resting place (and now Balsa Man), Baker Beach delightfully boggles urban expectations!
A great destination at any time, bright clear days are stunning. Parking is a bigger hurdle than towel space on the warm and sunny weekends when BB gets moderately busy - and overall the beach is surprisingly uncrowded. The north end of the beach is preferred by naked sunbathers - and the offleash area for dogs - plan accordingly :)
The beach is a great hike from the Golden Gate Bridge, the Presidio, or the Lands End/Legion of Honor direction (China Beach is similar/smaller) - adjoining Seacliff hosts some amazing homes (including former residences of Robin Williams and Sharon Stone), in addition to the usual commercial provisions. Bazaar Cafe is an easy walk and hosts (truly) independent live music in addition to food and beverage.
Website: http://www.parksconservancy.org/visit/park-sites/baker-beach.html
Wikipedia: http://en.wikipedia.org/wiki/Baker_Beach