When you are in SF try to get across the Gate to Ft. Baker. It has a refurbished Nike bunker. Not exactly a cannon, but still Artillery. Also, for fun, as you go across the Bridge, look at the hills on the Marin side and see how many of the artillery emplacements you can spot.
Don't forget Alcatraz, it was a Federal fort before it was a prison.
http://en.wikipedia.org/wiki/AlcatrazIf you want a day long trip, get up to Ft. Ross, north of Jenner on Hwy. 1.
http://www.parks.ca.gov/?page_id=449Be sure to watch the video on that CA parks site for a quick overview. As a note of interest, Hwy 1 used to come into the fort through the east wall, right in front of the chapel and then go out through the main gate on the west. It has since been diverted. Oh, and the CA site says about 2 1/2 hours from San Francisco - it lies, give yourself 3 1/2 (that is going up Hwy 101, and traffic on that varies a lot from hour to hour). More if you go all the way up old Hwy. 1 rather than Hwy. 101.
http://en.wikipedia.org/wiki/Fort_Ross