How fun! You'll have to say hello when you're here! Some restaurant ideas:
Now cheap is a subjective term, I'll give you my favorites and know that you can determine if they fit your budget.
Downtown: Pied Piper Bar in the Sheraton Palace. Strong drinks, truffled French fries. The hotel's high tea is a wonderful treat. Tadich Grill, oldest restaurant in SF. No reservations, the counter is great. Chowder, Louie salads, all their fish is fresh and amazing. Great staff. Ferry Building, loaded with great stalls. You can make up an amazing picnic for yourself for a bench outside or in your room (romantic). The wine shop, Cowgirl Creamery cheeses - get samples! Acme bread, there are 3 fantastic seafood places. Prather Ranch beef, the Italian Deli, Miette cupcakes, the chocolates. I could go on and on. Fun on a Saturday but crowded. Great gift shopping. Yank Sing for dim sum in the Mint Building (on Mission), amazing but not cheap. Henry's Hunan on Clay, cheeeeap, great Hinan, green onion pancakes, pot stickers, close to Chinatown and fun walking around there.
North Beach and Russian Hill: Capp's Corner, my old hangout when I dated the Sicilian. Cheap, great good, old SF place, lots of old photos, authentic, family style service. Washington Square Bar and Grill, it's back and better than ever. A chops and martini place. Rose Pistola, great grilled fare, upscale yet casual. Golden Boy pizza if you just want a quick slice. Stella pastry, nom nom. Old Victorian Pastry Co, my favorite pastry place, best cannoli in SF. My birthday cake comes from here, the St. Honore. Molinari's Deli, best Italian deli in SF. Get a prosciutto sandwich and a salami (they make it!!) and head up the steps to Coit Tower for a picnic. I used to do that a lot in high school. Mario's Cigar for tirsmisu or a coffee. The coffee place on Grant and Green, the first beat poet coffee shop, sorry I can't remember the name, it's on the corner. Zarzuela tapas on Russian Hill. Best tapas, these guys are authentic and my good friends. Across the street is the original Swenson's ice cream. I used to live right there and lived in those two places.
The Wharf: Nonnie Rose is great, Alioto's, Sabella's they are all great places. My ex was related to all of them and it's solid good fish. If it's a nice day go to the fishing boats, one of them (near the Wacky Jackie I think) offers an hour tour on the bay for $16. Best deal in town, the captain is cool, bring a beer and enjoy the views. I do this for fun when I play tourist, sometimes they let me pilot the boat.
Marina: A16 for dinner. The chef just left but I hear his staff is still keeping the magic alive. My fav Mexican is there, hole in the wall, Los Hermanos. Lucca's Deli, another good deli, good for a picnic by the Palace of Fine Arts dome.
Central Van Ness: House of Prime Rib, my family's special occasion place. Just the best prime rib. The 49ers regularly eat here, full meal under $30 per person and there is just no better. I'm going tomorrow with my parents! Super cheap: Tommy's Joynt, best hot dogs, hoffbrau, not the best food but decent, cheap, huge portions, thousands of beers, I go here quite a bit when I'm really hungry and on a budget. On Polk Street my fav seafood splurge, Swan Oyster Depot. Sit at the old marble counter, get a cold Anchor Steam beer or a decent chardonnay and eat anything on the menu. I have had itvsll and it's great. Last time I had a cup of chowder, oysters on the half shell, seafood cocktail, their special raw scallop dish (off menu). Their Louie is great. Bliss. Then a donut from Bob's up the street!
Japantown: Sushi boat is great! Very frsh, mostly Japanese eat there. Mifune for udon and the place to the right of it for ramen. The women's shop there has amazing scarves cheap, my favorite gifty place. In the back of the mall is the great knockoff handbag place.
By the beach is the Beach Chalet. Solid food, great beers made on site, view of our chilly windy No Cal beach, Ocean Beach. By the windmill and GG Park.
There are just so many places, many many fancy and expensive places I'm not even mentioning, they are the ones that get into the magazines and the places I've mentioned are where locals go. I'll post more as I think of them. I have to go snack now, I'm hungry now after thinking of all of these places!!!!!