I love the Haight/Lower Haight area.
At the end of Upper Haight is Golden Gate Park so you can start walking around it from there. And the BEST mexican food in all of San Fran is there too, Zona Rosa. It's on Haight and Cole and is amazing.
Amoeba records is on the next block and if you're a music person you'll have the time of your life. They have everything!
In Lower Haight (Haight and Fillmore) there is a fantastic art gallery, fifty24SF. It has mostly lowbrow art (my fave ) and other really good stuff too.
I'm a bit of a sneaker freaker so the Adidas store on Haight is where I spend a lot of time (and money ) when I'm there.
Bar wise Lucky 13 is very cool, it's on Market and 16th. The Castro is great, there is a deli on Castro and Market that sells the best flavored coffee, they usually have flavours like blueberry and vanilla - YUM!!
If there's anything on, try and go to the Red Vic. It's a movie theater on Upper Haight and shows some great films and has been there forever. It's a San Fran institution.
Is there anything you're wanting to do specifically?
Without a doubt SF is my favorite place in the States, it really has such a great vibe!