Fort Mason is a former military installation that has great views of the Golden Gate Bridge. Head waterside to the Fort Mason Center where you’ll find all manner of activities. The Presidio, a park and former military base, is very atmospheric and hosts numerous events. One of the best reasons to visit it is to see as many of the four Andy Goldsworthy Art installations you can. Visiting either the Presidio or Fort Mason when the gourmet mobile food markets are about is a particularly lovely thing to do on a summer or autumn evening (offthegridsf.com).
The Presidio began life as a Spanish fort in 1776 and alongside Mission Dolores is the oldest remnant of post-settlement San Francisco. Once a US military defence centre in WWII, these days it is a recreation area with many historic buildings and some residences. It’s about eight kilometers from Downtown.