Three nights, working, on the cheap. Please feed me. [San Francisco]
Depends on how little time you really have, and where you're from.
If you're not from SoCal, you can do worse than the In-n-out down there. Search the board for all the in n out lore, I just get a #1 meal.
Crab's in season. Grab one. If you're used to getting your hands dirty, just get a fresh steamed one from the sidewalk places, there's one by the big crab sign, just make sure you see a live crab go into the steamer. Not cheap, but a peak experience. Avoid the chowder.
The close edge of north beach isn't too far a haul, with places like Tony's Pizza (one of the few, if not only, coal burning pizza ovens on the west coast, but not cheap). I've had a good sandwich at Mario's Bohemian, where I think you get the Chicken Parm Sand witch.
I am not fond of Tommy's, it seems more nostalgic than good. It's a bit of a haul (but a straight shot, esp by bus) from FW. At least it's cheap.
Usually locals avoid the heck out of FW. At least you're a 5 minute taxi from quite a lot of food, so you should let people here know if you can relax the constraints on time or price - or add a little detail on what "cheap" and "quick" mean to you....
Not the best restaurant in North Beach, but one of the cheapest (and an old favorite of mine) is good old U.S. (for Unione Sportiva) on Columbus. Dinner specials from $10.00 and calamari and pasta or veg. (1/2 order, believe me it's plenty) on Fri. night for $9 last time I checked. Next door, fancier and a little more expensive, but still good value is L'Osteria del Forno. More upscale Northern Italian vs. U.S.' southern Italian/ East coast Italo' -American style. Another rec. for Yuet Lee, Calvin Trillin and I both like it.. Codmother on the wharf is the only place I'd consider eating, although I've heard that Tanguito is also okay.
Take the Van Ness Bus or Polk Bus. (To save even more $, walk Polk Street, it's mostly flat and some of it you can window shop.)
Get yourself to Tommy's Joynt on the corner of Geary/Van Ness (one block over from Polk). You can get filled up for very little money. Hofbrau type place. I like the turkey.
Also on Polk near Broadway there's an Escape from New York pizza by the slice place -- really big pieces. They have the regular flavors but a special one is the sliced red potato with roasted garlic and pesto.
You can walk to North Beach, go to Green St. and get a slice of Golden Boy pizza. (It's square pieces, they'll heat up the slice for you.) If you like coffee, go to Trieste on the corner of Grant/Vallejo.
Chinatown is next to North Beach, lots of cheap places but the one I prefer costs more: Yuet Lee, corner of Broadway/Stockton.