Nick, if you have free lodging in San Francisco, I'd think about just staying there. In September, San Francisco may well have the best weather in the state. It has an excellent public transportation system, and there is so much to see and do.
Still, the Monterey-Carmel area is wonderful any time of year.
Too bad the Festival of the Sea in San Francisco is in August (Aug 19) this year. It's one hell of a wonderful music event.
-Joe-