I will recommend San Francisco, since there are lots of places you can visit. Such as city site sign, fisherman's wharf, Golden Gate park, various museums. Also, if you like natural view, you can go to Yosemite National park, go to Napa for winery yard if you enjoy tasting wine. Anyway, SF is always my favorite city. I went to San Diego three times, and went to the Colorado Island, La Jolla, SD zoo for panda, etc. SD is also nice, but SF is the best.