Given its population density and metropolitan ambience, San Francisco fits the "city" description better than any other place in California. Its topography, location (on the tip of a peninsula), and scenery supplies innumerable views to behold (a paradise for amateur and expert photographers). There are many fun neighborhoods, from the chic Marina District to the eclectic Haight to bustling Market Street. As with any city, there are some downsides. Here are a few: lots of transients, driving is a chore, and finding a place to park is a frustrating experience (and can be very expensive).
Things to see (or do): Golden Gate Bridge, Alcatraz, Marina District, SBC Park (Giants Baseball), Cliff House, Market Street, Golden Gate Park, Little Italy, North Beach, etc.
Things to pass on: Cable Cars, Mission District (rough area), and Candle Stick Point.
I still have much more to see. Have fun.
Quick Tips:
When making a day trip for downtown shopping, park at the Fifth and Mission parking garage. It is in a great location and is fairly priced, safe, and easy to find.
Best Way To Get Around:
Driving may be the best if you can deal with hills and do not mind searching for parking. Muni and BART good ideas. Downtown taxis are plentiful.