Angel Island is located in the middle of the bay, just north of Alcatraz. It’s now a state park, but its past is layered with military history. It was a military post during the civil war, an immigration station, a WWII Japanese internment camp and cold war missile site.
Most of the structures related to these military endeavors are abandoned. But the park itself is open for business.
In addition to the military sites, you can hike nature trails and get 360’ views of the San Francisco bay.
Tip for visiting: Take the ferry from pier 39 and rent bikes on site for a self-guided tour. Bring a lunch and plan to spend the better part of a day exploring the island.