A trip over to Alcatraz Island is a must-do when visiting San Francisco—although I would recommend you book well in advance—and is well worth the money. We boarded the boat at Fisherman's Wharf and had a lovely 20-minute cruise over to the island (even though it was the end of October, it was very much like a British summer day!). When we alighted at the other end, we had a bit of a trek up the hill to get to the actual prison (if your mobility is limited, investigate what help they give, as it is STEEP!), where we were given an audio tour in whichever language we needed. So we put on our headphones and wondered around the historic prison where notorious criminals such as Al Capone were held in tiny little cells stacked three high to the ceiling. They have reconstructions of what cells looked like when they were fully operational, a trip to solitary confinement, and the dining room. All the time you are wandering around, the narrator is telling you about all the escape attempts and people who died in the very room you're standing in!
The views from the island are truly magnificent, which gives you the idea of the torment prisoners must have gone through knowing that civilisation lay a short, shark-infested swim away!