The Presidio is a park run by the government that has been used as a miliary outpost sinc ethe Civil War. Lots of greenery and trails.
For nude sunbathing stay NORTH of this sign, in the area you see, and enjoy the million dollar view of the bridge and harbor!
A walk along the beach by Chrissy Field was a bracing experience. The views of the Golden Gate Bridge are just breathtaking.
I don't remember know the name of this place. It was a long drive, a long walk, a steep hill. But it was worth every single minute.