Hi Jools, quite a number on our tour went to Alcatraz on the afternoon and hadn't booked previously; although in high season maybe it might be different.  I didn't go, as we did see it close up and had a good commentary on the Bay cruise that morning.  A group of us went around the city instead - on the cable cars, through chinatown and little italy, with a stop off for a drink along the way - great fun!  Enjoy the hol, it's fantastic in every way!