499 Castro St (between 17th St & 18th St)
While obviously this is something that those who may be GLBT may like, I would also invite others to experience the history here. There is a little museum here that is truly a gem. If you saw, and enjoyed, Milk, I really think you will appreciate this as well. Explore the history of how the neighborhood turned from a very Irish-Catholic neighbhorhood to what it is today. You can see so much memoribilia, including the buttons from the Coors boycott as well as the suit Harvey Milk wore when he was assasinated. It is truly touching and haunting all at the same time. I feel very strongly about this place. Admission is only a few dollars and it is well worth the stop.
Also, since you'll be in the area, Harvey's old store, Castro Camera is just a little ways up. Castro Camera is defunct, but if you want to see the building, it is still there.