Make sure you ask about the nuclear wessels.
I lived in SF for about 9 months or so, and I loved it. It's difficult to live because it's a very expensive city, but it's also a gorgeous city.
I lived in Haight-Ashbury, which is the original Summer of Love hippy zone. Basically the entire area lives in a permanent pot cloud, with charms and tie-dye as far as the eye can see. The main road through the area leads you directly to Golden Gate Park, so you could combine the two into one day. I would suggest the Pork Store Café
on Haight for breakfast or lunch, except that I can't speak for any possible vegetarian options.
Great walks that I used to enjoy were up from Haight over Twin Peaks and back down Castro, or all along the Embarcadero to Fisherman's Wharf.
If you're going drinking in the Castro, I used to like The Mix
on 18th St or the Pilsner
If your shopping takes a more pervy bent, my boyfriend swears by Mr S Leather
in the Mission (link NSFW).
You won't find a Target in the city, as I understand there's some kind of rule where big chain stores like that have a hell of a job getting permission to open within city limits, in an attempt to keep the shops more exclusive and individual. Of course, there's still an Apple store. And Berkeley definitely has all that stuff.
Oh, and if you know what you're looking for, you'll see a porn star on every corner too.
I'll come back if I think of anything else. Hope you have a great time!