There are many barns in that area. The ones I would recommend are Stanford Red Barn, which has a great community lesson program, and any trainers at the Horse Park at Woodside. Both of those might be a little far south for you, but they are fantastic places. Clermont Equestrian is a little closer to you and their trainers are also quite reputable.
I'd recommend looking at bayequest.info for barns close to you as well Best of luck!
My parents live in Napa, so I know stuff there-- Four Star Farm owned by
Natalie Rooney-Pitts. I took a few lessons with her and she was FANtastic. A bit far for daily stuff but doable on the weekends. PM me if you need more info-
And the wise, Jack Daniels drinking, slow-truck-driving, veteran TB handler who took "no shit from no hoss Miss L, y'hear," said: "She aint wrapped too tight."
Shamrock Ranch is the only place within 40 minutes of SF (it's 20 mins to SF from there) that has pasture board that's any good (their largest is 30 acres, compared to the much more overpopulated dirt lots people call pastures in the Bay area) They have stalls, arena, hot water, Western trainers onsite and Grand Prix dressage trainer who visits monthly and also does jumping. Cool mix of boarders, not fancy but relaxed. Horses are usually available to part lease from their owners. Only one kid board there, so the atmosphere is calm.
Park Pacifica has a massive covered arena, no turnout to speak of and some lesson programs/horses to lease.
From there you'll travel 10-20 mins down the peninsula on the coast side to several similar options in Moss Beach and Half Moon Bay (Moss Beach Ranch, Moon Valley Ranch, Renegade Ranch, Ember Ridge Equestrian Center and a bunch of others: https://www.bayequest.info/Business/showBusi_8_11.htm )
If you take 280 down the middle of the peninsula, it's 30-45 mins to a lot of more expensive barns and generally hotter weather.
James Alliston is in Castro Valley, right down the street from me, at Graceland Equestrian Center. He's the eventing guy that just won at Galway Downs this past weekend. He is British, lives in America, young. I have seen him school his young horses at the local shows. He has quite the following from what I understand.
Check out the Sunshine Valley Road area in Montara, just south of Pacifica. I used to keep my horse at a small co-op boarding stable there. Great trails through the community and around a commercial flower farm. I kept my horse at KamRun Ranch and rode with Charles de Kunffy, not sure if the barn is still there but worth driving around and asking what's available now.
Lots of barns on the Peninsula, where you will find a good selection of trainers. And you could also check into Half Moon Bay and see what's there.