Civic Center lunch?
Lalita, on Golden Gate and Hyde(?) has good Thai food. The burrito place on GG just east of Larkin (Castillito) has good burritos and tacos. Wrap Delight on Larkin has wonderful bahn mi and good spring rolls. Larkin Deli Express has pretty good sandwiches, good meat and potatoes sides and deli salads, and rockin' chocolate chip cookies. Mangosteen, already mentioned, is quite good. Gyro King on Grove has yummy middle eastern food. I'm also quite partial to Ananda Fuara at Larkin and Market (woowoo crunchy vegetarian food, but very good). The Wed. farmer's market has a good tamale truck.
Welcome to the 'hood. There's lots here, it just takes a little poking around.
All-Star Tamales has a stand at the Wednesday Civic Center farmers' market. It can be fast or excruciatingly slow depending on what time of day you get there. Try to go for an early or late lunch.
Your best daily bet is a $2.50 sandwich from any of the banh mi places mentioned by Sue.
Turtle Tower is also a trek, but very good if you want pho ga (chicken soup and noodles). Hot, clear, lightly seasoned broth with tender chicken pieces and wide flat noodles.
If you're looking for fast and cheap, I'd hit one of the banh mi places, like Saigon Sandwiches (560 Larkin) or Wrap Delight (426 Larkin) and eat your sub in front of the Asian Art Museum, if it's nice enough out.
And it's a bit of a hike, but Mangosteen (601 Larkin) and Bodega Bistro (607 Larkin) are both decent sit-down places for Vietnamese, and not too pricey (though not super cheap either).