San Francisco: Alameda Island
Alameda Island, which holds most of the city of Alameda and the giant, defunct Naval Air Station Alameda, is not exactly a rustic getaway. The island features several massive and operational docks and is most famous for its abandoned military zone, where parents from Oakland and the South Bay come to teach their kids to drive on empty roads between former military barracks. This place is a giant industrial playground. There is no better place to race bikes or motorcycles, launch rockets, or run through abandoned neighborhoods. And when you're done flooring it or watching the MythBusters guys blow stuff up on abandoned runways, there is an urban winery where oenophiles can sample locally-made reds while sitting on a porch overlooking San Francisco and a the broad bay to the north of the city and a grand old movie theater. It's the best place in the Bay Area to remember how central the area's maritime legacy still is to the city's identity.
More information: Alameda Island is reachable by three bridges from Oakland proper as well as by the Posey Street and Webster Street subways. The naval base is open, but areas are sometimes closed for construction, deconstruction, or chemical clean up.