Mission District is nice, stay off of Mission Street itself though unless you love Mexican food authentic style. Beautiful parks and old churches to see, with hip little cafes, even in back alleys.
Market Street quickly goes from posh to scary, so as soon as you sense it's spooky, either take transit or a cab outta there!
H-Ashbury is kind of boring, reminded me of a mix of c.1997 Queen St West (U-Bathurst) and the Annex--aging hippies and students meet grungy clubbers. Blah.
I liked the upscale Cow Hollow area--sort of Laurier (Montreal) meets the Eglinton way, only denser and more interesting. Chinatown is ...well astonishingly old and Chinese.... Fisherman's Wharf is a lame tourist trap, but is worth going there for an In-N-Out "animal style" burger and to see the old wharves. (Which make you depressed realizing Toronto knocked all their old wharves down.)
I'm slightly biased--I liked the Hayes district as well. Sunset is very posh--and beautiful.
There's more to see...let me think about it (look at all the photos I took when I was on tour there last summer.)
If you rent a car, I recommend taking a tour out to the Lost Coast area--west of Petaluma (which in itself has nice relaxed historic downtown.) Places like Valley Ford, Bodega, Sebastopol, Russian River--spectacular imho.
One final tip: You must drink some "Eye of the Hawk" ale--some of the best I've had anywhere!
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