Seconding what Jeff said about relying on movies for geography: in
EdTV they show a scene of him running down a street in the Marina, down an alley, & then coming out of the alley at the Castro theatre. Erm... who knew that alley had a wormhole that led a couple of miles south?

They also have the ticket seller asking if he wants to scene the film in theatre 1 or theatre 2; the Castro is a single-screen theatre.
In
The Bachelor they have the love interest going to the TransBay Terminal (buses) to take CalTrain (train that runs south along the Peninsula) to Santa Rosa (north across the Golden Gate). *boggle*
The hill from which the linked picture was taken is Corona Heights. The museum which sits on its side is the Randall Museum, a 'discovery'-style museum. One side of Corona Heights is dug out, as it was the site of a quarry in the 19th century; the workers lived in the Castro which was then called Eureka Valley, or (because of the nationality of many of the workers) Little Switzerland.
Look for many pictures of fog in The City (btw, always capitalize it thusly) to get it right; it's really something you have to see to get. I lived in the Castro when I first moved to The City, & the ebb & flow of the fog around the top of Twin Peaks was a daily thing to watch in the summer. In the morning, the fog might have spread across the central hills & blanketed the entire city, but as the day wears on it would usually burn off to the point where we on the eastern side couldn't even see it; by late afternoon it would be a thick mass hovering around the edges of the hills above, spilling around the lower edges & drifting around us as we lay sheltered in the lee of the main heights, knowing that soon it would overflow even those to bear down upon us. On days when the fog pulls back to the ocean, it might lay only a few feet offshore, with small eldritch wisps breaking free to drift across the seaward cliffs, scouting for the invasion to follow. Those same days, tourists in the 'Disneyland' that is Pier 39 might see a thick stream of fog pouring through the Golden Gate & north along the Bay, following the lowlands of the Delta toward the Great Valley, where it would bring them cooler temps while we sweltered in the unbearable 80's, denied the natural AC to which we're accustomed. I've seen pagan renaming of the months here that start with June as 'Fog Rolls In', then 'Fog Hangs Around', then 'Fog Hangs Around Some More'.
Foggy days can be quite cool, & it's not just the wind but also the damp & the lack of sun. Without fog (which usually means 'temperature inversion), summer temps may be 80's or above, & without air conditioning that's rather uncomfortable at night. With fog, the daytime temps would be more likely low 70's, & could easily drop 20 degrees or more at night. Interesting enough, the coldest nights are ones without fog, as it provides a bit of a blanketing effect to reflect heat radiation back down; a foggy day followed by a clear night is the worst.
San Francisco claims to have more restaurants per capita than any city in the world; it's quite a believable claim. Adventurers should encounter sushi, Thai, Italian, Mexican, Peruvian, Bangladeshi, Indian, seafood, French, Ethiopian, Morrocan, Middle Eastern, Chinese, & many 'American' varieties.
Know your neighborhoods (whether you use the real ones or not). Some cover major sections of The City; others are so small they hardly seem worthy of an appellation but don't dare call it by the wrong name to someone who lives there. Every hill has a name, & many hillside residents will use <hill> Heights as a name for their area even if everyone else refers to that block as just a part of one of the more widely known neighborhoods.
Market St. is the 'Main St.' of San Francisco; it runs diagonally from the Ferry Building in the Embarcadero southwest toward Twin Peaks at which it starts to curve & jiggle to follow the slope of the hill. Streets north(west) of Market run on a typical n/s & e/w grid; south of Market they run on a skewed grid that parallels Market until you hit Van Ness just north of the bend in Mission St. (which is the parallel street south of Market for that portion of its length, then curves & runs south through the large neighborhood which bears its name). Intersections along Market are typically 6-way west of Van Ness, as you have the diagonal of Market bisecting the grid intersections there; east of Van Ness where you have to opposing grids on opposite sides of the street they're more often 5-way, with the angle of a grid northwest of Market hitting Market & the single intersecting street on its grid to the southeast.
If your players are mature enough, send them through the Castro. I don't think you capture the flavor of this city if you avoid part of its culture because it is so different from the 'middle American' experience. You could have a contact meet them for brunch at Cafe Flore, a nice little tin-roofed & glass-walled cafe that sits in an angled intersection on Market with outside tables. Wood-&-glass walls separate the tables from the sidewalk, with overflowing planters along their outer sides creating the illusion of a little garden oasis in the middle of The City. I recommend the three chili fritata (Mexican-style scrabbled-egg omelet thingie), or a chocolate croissant for those with lighter appetites.
How to describe a walk along Castro? Well, the Castro theatre should be the main visual focus due to its prominence; there are restaurants & bars & clothing stores like any commercial neighborhood. I'd suggest that you emphasis a lack of 'shocking' or 'freakish' without a whitewash of diversity. Guys holding hands, girls holding hands, friends greeting each other (& most of us
at least hug friends hello; a light quick kiss is fairly common). Mostly just people going about the business of their lives. In all the 'wow, this is rather more dull than expected', perhaps a few eccentricities such as someone with oddly dyed hair, or a ton of piercings -- & they might just as likely be straight as not. On a weekend night, a drag queen or two (the Castro is not a major drag haven, although Harvey's at Castro & 18th is bringing a welcome (imo) breath of drag with benefits they throw). In front of Daddy's (bar), a few leathermen standing around smoking (btw, that's something you've got to get in -- because there's no smoking indoors in California, sidewalks in front of every bar are packed with smokers; the air immediately outside of the bar is worse than the air inside by far). In the early evening, people heading to the gym, & people heading home from work. Buskers from the School of Performing arts playing string quartet in front of Cliff's (the hardware store) at night. If you were basing it a few more years back, you could have Cosmo Lady playing her accordian & wearing her winged Viking helm that had the flashing red lightbulb on top of it (she hasn't been around for years; was a cool Marin housewife who'd drive down to The City to play & sing in a lovely ethereal voice).
While most of the clubs in the Castro are bars, 3 have dance floors & draw a more diverse crowd (i.e. straights as well); this is becoming more common lately I've noticed:
The Cafe (formerly Cafe San Marcos) is upstairs above a tatoo parlor & Subway Sandwich shop near Castro & Market (crowd young, emphasis on the young -- did I mention young?) with a balcony overlooking the gas station at that corner & a patio in the middle where smokers can step 'outside' to legally light up. No cover. Draws more women than the others (the old Cafe San Marcos was the last lesbian bar in the Castro), & if their bathroom is full they'll march right past the line of urinals in the men's to get to the lone empty stall in the back (not that we care, really -- bathrooms in the Castro tend toward unisex).
Badlands has been the premier (imo) club in the Castro so far this century; what used to be an old 'country western dive' decor bar (with rock oldies music) has been remade into a modern dance club/video bar. $2 cover & 2 drink minimum (like they check); ID is always checked at the door (& is actually at all 3 of these). Lots of chrome, disco ball, round dance floor against the back wall with railings to separate it from the rest of the bar, bar itself runs the length of the place, side room with some of the old wooden benchs along the walls from the previous incarnation (used to have a pool table there in those days).
The Bar on Castro (or just 'The Bar') is the newest trendy hip-hop spot in the 'hood & definitely giving Badlands a run for the money. It's a very small bar, built like many of the local bars very long & narrow as it runs the length of the ground floor of the old Victorian which houses it. The 'dance floor' is just the back section near the dj booth; on weekends it's more like the 'sway floor' as it's just too crowded to dance (though some drunken idiot will always try, bumping into everyone around). Hip-hop Tuesdays are big. No cover. Red walls, front 'patio' (room open to the front with glass doors that close to separate from the true 'indoor' area, so that smoking can take place), bar on one side with the other having this cool wavy padded bench & little cocktail tables hanging from the ceiling in front of the bays formed by the waves.
What else about my neighborhood? There are gay men (like me) who consider the Castro 'our neighborhood' even though we no longer (or for some, never did) live there; there are also gay men who rarely if ever set foot there. Most of the major dance clubs are in SoMa, but I avoid those because a) too loud, b) too expensive, & c) too many drugs.
Oh, drugs are definitely common everywhere in The City; you can pass someone smoking pot most any day, X was (is?) definitely big, & crystal continues to be a major problem for some (not labeling all users as abusers or anything).
Ah, be sure you get your terminology right -- cablecars are the 'trolleys' that run on 3 lines: Powell-Hyde, Powell-Mason, & California; they're cablecars not trolleys (I used to get admonished for calling them the wrong thing). 'Trolleys' are the refurbished old streetcars that run along the surface of Market from Castro to the Embarcadero & around to Pier 39. The underground is 'the Muni Metro' or more commonly just 'the metro'. Buses don't actually have schedules, just 'frequency of service'; you can sit through an entire segment of time matching two 'frequencies' for that time of day & then see three arrive back-to-back. At rush hour, the metro can be so full by the time it reachs the third stop that you have to ride backwards to the start in order to get on one heading out; every car that continues along the main line to Castro (3 of 5 lines) will empty of half of its passengers there.
Houses in the eastern part of The City are mostly Victorians; bonus points for including details of whether any featured house there is a Queen anne (peaked roof, often a turrent), Edwardian (flat roof), or Italianette (Italianate? haven't seen it spelled -- like an Edwardian with a faux top coming up in front of the actual roof). Further west you see more of a different style, more square & stucco, for which I've forgotten the name (this would be most of the housing in the Sunset & Richmond, which is the 'partner' of the Sunset north of Golden Gate Park; it also features an 'Inner' & an 'Outer').
Creepy places? Fort Battery & Fort Funston are old gun-placement locations along the western seacoast; though not big, each placement has one dark damp rooms to each side of the short central passage through (many of those passages having trails leading through them). Other than that, I'm not sure; my local D&D group is talking about a Vampire campaign & we've been trying to think of places where we could actually meet.
OH! For a place you couldn't easily get in reality (but adventurers, ah!), there's an abandoned metro station past Castro as the trains start the long run under Twin Peaks -- Douglass St. Station, I think it is. You can see the old platforms covered in graffiti as the trains pass through; perhaps there are other places out of sight in forgotten corners back there? The old access for the trains was to go on the surface along Market St. & then go underground right before that station; the tunnels that led down are still there but surrounded by fencing -- or adventurers could just walk along the tracks from Castro station, with the inherent danger of a TRAIN COMING!!!
Golden Gate Park & most of the western part of The City was originally all sand dune; if you're concerned with accuracy, avoid caves or such out there & keep them around the hills (which are mostly granite & not that likely to have caves either, I guess, but at least they could support worked caverns).
Whew! Rather more than I'd thought to write, & it's late. Hope this helps.