I find that walking the neighborhoods interests me the most. Walk around the Castro, Haight/Ashbury (right next to Golden Gate Park), the Mission, etc.. I love love love Valencia St. between 16th and 24th (beyond there is good too, but these streets are my daily walk and I love it). There are a ton of used bookstores, cafés, vintage clothes shops, taquerías, etc. It’s not a touristy area so you’ll get a true feeling of the city on this street.
I find the touristy areas (such as Pier 39) to be boring. Tropical-themed restaurants and lame “funny” t-shirts don’t represent San Francisco. What sets this city apart are the people in their natural habitat, the palm trees, the beautiful houses, and the hills. Definitely walk up some hills(bring a water bottle)!
Definitely check out Golden Gate park (it’s really big!). On Sundays the roads are closed to traffic so if you have bikes or roller blades or whatever this is the time to bust them out.
Definitely eat Mexico food while you’re here. In the Mission. La Cumbre (16th and Valencia) and Can-Cun (Mission and 19th) are both really good. And get Sushi.
Chinatown’s cool, but it’s no different from the Chinatown of any other city, so if you just have a weekend it’s not essential.
If you see Street Musicians, give them money! I’m here for the winter busking, and although I’ll be gone by then, my fellow street musicians will still be out and could use donations from friendly tourists:)
Have fun!!