San Francisco

castro
definitely go see something at the castro theatre
if you don't have a bike, catch the f-market (cool deco street cars, just for the heck of it).
if you do have a bike, watch out for the streetcar tracks around town-- total landmines and if you're not careful they can wipe you out.

mission:
+1 for tartine (almond croissant to die for), and they've a fancy restaurant by the same owners if you can afford to splurge.
bi-rite has some pretty damn tasty treats, and the ice cream parlor they've opened across the street is undeniably fantastic (though mitchell's still my personal favorite; see below)
if you're at zeitgeist you can have bbq or skip the bbq and wait for virginia to come 'round with her homemade tamales.
mission burritos have gotten pricier than when i lived there, but they're still pretty irresestible. eat as many as you can, and weep for them when you return to london.

my favorite tacos are from the taco window on 21st st (near either treat st or harrison st-- can't remember)-- the place has a name that starts with the letter c, and which i can't recall, so just look for the big yellow awning thing attached to a house, walk up to the window and order... i'm salivating just thinking about it.

find yourself a nice salvadorian restaurant/pupuseria, too-- i like el zocalo on mission/29th street (there are other good places, but el zocalo is open conveniently late). mitchell's ice-cream parlor is nearby; not really anywhare to sit, but the ice cream is amazing.
i think they still serve mitchell's at st. francis fountain on 24th street/york (definitely worth a visit).

bernal heights is nearby, too and has great views (and a vegan ice cream parlor, among other things).

north beach:
yes, a cable car ride is worth it, but only if the car is half-empty, and the driver goes nice and fast.
just nearby, is coit tower (dedicated to firemen) and the WPA murals. good views all around. wander around the little stairs that dot telegraph hill, and look for the wild parrots (cousins of the ones in london).
+1 for city lights. while in north beach, go to 101 records; you may or may not find what you're looking for, but i really like the place.
definitely have canoli (has anyone ever seen canoli in london?)- stella bakery is really good (nice cookies), but there's a place across the street that has good canoli, too.

fisherman's wharf
definitely skip fisherman's wharf-- there's nothing at all to see there, except the sea lions.
in fact, i do like fisherman's wharf, but only in the wee hours when it's just you and the barking, blubbery sea dogs.
if you want clam chowder/seafood, go to sotto mare on green street (next door to a good pizza place) in north beach (sotto mare is pricey, but delicious)
OR if you like ersters, go to swan oyster depot on polk street (this is not north beach, but sort of like btw tenderloin/nob hill; yes, the tenderknob). go there for breakfast so that you miss the lunch rush and instead can watch the fresh catch being brought in big wooden crates, while you enjoy having the whole counter to yourself and can linger over your coffee and oysters (sounds gross, but tastes like being at the beach).
oh, definitely go to baker beach, or maybe ocean beach if it's just after christmas-- there are always big x-xmas tree bonfires at night.

*the richmond *
on the way out to the beach, cruise through the richmond and check out clement street for an abundance of pretty much any kind of east-asian food you can imagine (though the best vietnamese tends to be around the tenderloin).
also, on geary blvd there's a russian bakery with a red/white awning that has good (greasy but tasty) blintz like things filled with meat or sweet cheese, and there's a huge eastern european deli across the street (also on geary) that has all sorts of good stuff-- i'm partial to the fresh refrigerated pickles in the back. there are some interesting high-alcohol content beers back there, too.

there's tons i've not yet mentioned, and yes, this post is all about food but i can smell supper...