If I knew I would only have time for one attraction in San Francisco, Alcatraz would definitely be in the running for my one pick. From the size of the crowds gathered at the ticket booth every day, I'm probably not the only one thinking the same thing.
The most important thing to know about going to Alcatraz is you need to order tickets early. Order them before you leave home. In fact, depending on the time of the year of your visit, order them way before you leave home. I've gone by the ticket booth before and have heard announcements that the next available trip to Alcatraz is in 3 days. So, check out www.blueandgoldfleet.com to make sure you get a ticket.
When you're reserving your ticket, seriously consider the "Alcatraz After Dark" tour. It's only offered on certain days and not all sections of the daytime tour are open at night, but you'll get a much different perspective of the island and what life was like on The Rock. Plus, you'll get some incredible sunset and evening views of the San Francisco skyline.
While you're on the tour, there's a couple of things to watch for. First, when you're in the main part of the cellhouse, look for the desk the National Park Service (NPS) has set up offering different Interpretive Programs. These are always really informative and, depending on the tour, may give you an opportunity to see parts of the cellhouse others may not get to see. The program that visits the hospital was really interesting and, after dark, kind of eerie.
Second, when you get to the part of the tour where an NPS guide offers to lock you in a cell in solitary, try it. Just stepping inside the cell is enough, but having someone lock the door behind you gives you a very real feel of what it was like to live behind bars.
Before you go, also make sure you check out www.nps.gov/alcatraz/. The Park Service has a really nice little site going here. It goes into great detail about the history of Alcatraz as well as how to get the most out of your trip.