Even the most hardened traveler will enjoy Alcatraz. I know that my companion didn't want to go, but once we were there, she had a great time.
The boat ride is part of the tour, and it's a nice introduction to the San Francisco bay. If you take a late cruise, you'll see a beautiful sunset over the ocean.
Alcatraz was once burnt down, in the 60s. The main cell building is still intact, and this is where you'll pick up your self-guided tour. All around it are the burnt shells of other buildings, now inhabited by wildflowers and seabirds. The view from the island and the mix of nature and man-made structures is a photographer's dream.
The self-guided tour is quite interesting, peppered with voice recordings of actual inmates and guards. You can almost imagine yourself here, especially out in the "yard". Sit down on the stone bleachers, and imagine that you're locked in here for life, with the lights of San Francisco just on the horizon, forever tempting you.
Learning about the Indian occupation of Alcatraz Island shouldn't be missed, and you can catch a brief film on this topic near where the boat launches.
You take a blue and white ferry from Fisherman's wharf, and they do recommend that you reserve in advance. We did, which saved us the hassle of waiting in line. You can also go watch the sea lions off the pier while you wait for your boat.