Valencia Street (The Mission)
The Mission is a great area and worth exploring all over, but Valencia is a good place to start. If you take the BART to 16th and Mission, Valencia is one block up the hill. There are tons of restaurants there (highly recommended are Sunflower Vietnamese Restaurant at 506 (get the lemongrass chicken!) and Mission Cheese at 736), bars and some great thrift stores. You might also want to check out 826 Valencia, the flagship location for Dave Eggers 'creative writing scheme for kids which is now nationwide. The front is a pirate store and lots of fun to wander around.