I highly recommend the audio tour of the cellblock. Very informative and very interesting too. I would recommend that you do that first. Also, be sure to listen to some of the speakers while you are there too. They usually have a couple of them every day. When we were there, we listened to a guy talk about the Birdman of Alcatraz for 45 minutes or so and it was really, really interesting!! We spent at least 5 hours on the island. After the audio tour, we spent lots of time just walking around the place...after seeing so many movies about Alcatraz, it was fun to just be there and soak it all in. Before you go, definitely watch Escape from Alcatraz again so it's all fresh in your mind
