I have been to the USA about a dozen time (on business and holiday) and San Francisco is the nicest place I have been to in the USA.
Many US cities have pretty awful centres (downtown), with large skyscrapers and are dead after work and at weekends.
But San Fransisco is a REAL city, with loads of nice victorian wooden houses and as someone has said, it is a real WALKING city.
It does have its toursit parts (Fishermans Wharf for example) and that it a great place to stroll around and people watch.
And there is also Alcatraz Prison, Golden Gate bridge, Chinatown, Golden Gate park, the trams and so on.
The people in SF are also frendly, and I think you could have a great holiday in JUST San Francisco.
The weather in SF is also good. Because it has water all around it it never gets too hot in summer, or too cold in winter. It has an nice steady temperature all year round.