Go west from the Golden Gate Bridge and through the Marin Headlands to find a history lesson on San Francisco's war preparedness.
This location houses a series of WWII gun emplacements. They were built to protect the US from Axis battleships (which never arrived) and they were re-purposed during the cold war to deflect aircraft missiles (which also never arrived). You can hike through the tunnels and base stations.
Tips for visiting: You’ll need to get to Marin Headlands by car. You can make a day of it by also visiting nearby Stinson beach and Muir woods.