Just minutes from the busy city, across the Golden Gate, is Fort Cronkhite in the Golden Gate Recreation Area (the largest urban national park in the United States). Formerly a US Army installation, it's been part of the GGNRA since the 1970s. It was saved from a massive development scheme that would have had the hills covered with homes and shopping centers. On a recent visit, I encountered a few deer and a coyote while hiking the beautiful trails above the ocean. The place has everything from historic buildings to black sand beaches. You can tour an old artillery battery, visit a Cold War Nike missile site, stop by the Marine Mammal Center or walk across a suspension bridge to one of the oldest light houses on the west coast, which was completed in 1855. You don't even need a car to get there as it's easily accessed by bike or, on the weekends, MUNI bus service (route 76) to the Marin Headlands.