Alcatraz is the most famous island prison in the world and one of San Francisco's must-see attractions. The former maximum-security penitentiary sits on a small rocky island in the middle of the Bay, surrounded by cold, swift currents. It held inmates including Al Capone and Robert Stroud (the Birdman of Alcatraz) from 1934 until it closed in 1963.
Today it is part of the Golden Gate National Recreation Area. The audio tour of the cellhouse is excellent -- narrated by former guards and inmates, it brings the place to life in ways a guidebook cannot. Book your ferry tickets well in advance through Alcatraz City Cruises, as tours sell out days or weeks ahead, especially in summer. The night tours are particularly atmospheric.