Yep, and have, kinda :-P I wasn't by myself though - my DH was flying into Chicago, the city my best friend lives in. So we flew over togther (albeit him in Business, me in Economy! Stupid United wouldn't let us swap seats haha) had the weekend with my best friend and her DH, he worked for the week (out of the city so I didn't see him at all except for one dinner together), then we had one more day together and flew home. We had about 10 days notice too...
San Fran is a GREAT city. You won't have trouble entertaining yourself day or night! I'd even try to book myself on a day or overnight tour up the the Napa/Sonoma valleys :-)