Today I went hiking above Fort Cronkhite, in the Marin Headlands North of San Francisco. There is a former Nike Missile Site on Wolf Ridge, which is about a 2 mile hike each way, with 900 feet of elevation gain. The weather was quite nice for November, and the views were spectacular. Jef Poskanzer has a fantastic web guide to the Nike Missile Sites of the San Francisco Bay Area. Ed Thelen's site also offers a ton of detailed information on Nike sites.
While many of the Nike sites are rather barren today, there are a few that still have interesting buildings including Wolf Ridge, Sweeney Ridge, and Fort Barry. Fort Barry has been restored, and offers tours on the first Sunday of each month. Apparently you can even ride the bomb elevator!
For those interested in scouting photo locations, there is a really great map and guide to the seacoast fortifications of the Golden Gate. One side features a large map showing the type of weapons that were used at each location. The locations are also color coded by time period, from the Spanish-Mexican era through the Cold War. This great resource cost an exorbitant $1.50 at the Golden Gate National Park Store in the Embarcadero Center.