There are lots of stores in the area. Check out the United States Yarn Store Map in another thread.
I recommend Article Pract and The Knitting Basket in Oakland, and Stash in Berkeley. Yarn! in Alameda is also nice.
You'll be just 15 min away from SF by train, so you can explore stores there, too. (Though, sad to say, I've yet to do that. And, I live here!)
Enjoy your trip!