Give Sara's Ruby Mild a shot at Magnolia, my favorite beer there. If you are renting a car you NEED to check out Russian River Brewing Company. It is a 45-50 min. drive north from the city to Santa Rosa. Get some Fresh Pliny and specatular sours. The food is really good as well. There is also 21st Amendment and Anchor brewing, but you need to make an appointment to go on a tour at Anchor. Lagunitas is also north of the city in Petaluma.
The San Francisco area is one of my beer meccas. I love that place!