caption: Slide 2 of 51: One of the most visited American tourist destinations and among the coolest things to do in San Francisco is <PERSON>, which was once a fort, a military prison and a maximum-security federal penitentiary. There are a couple ways to take in the spectacle -- you can go on a self-guided tour at your own pace, join a tour group or attend any of the free interpretive programs the National Park Service staff and volunteers offer throughout the day. According to the NPS, at least two to three hours should be allotted to fully visit the island and considering the ferries run every half hour, you can return to San Francisco without any fuss.