The Golden Gate National Recreation Area includes many scenic areas which used to be controlled by the military.  From Fort Cronkhite, located in Marin County, you can hike to the summit of Hill 88.  This hike will reward you with spectacular views in all directions, and a healthy workout (5-6 miles, 1000′ elevation gain).  The abandoned, dilapidated & graffitied structures which reside at summit were once part of a radar station for the former Nike Missile Base which is located nearby.