Private, clean, discreet and cash based. You get a room with a hot tub, a shower, (usually) a sauna and 'yoga platform' that looks an awful lot like a bed.
My favorite: Hot Tubs on Van Ness in the city. Nice looking, enough rooms and desk staff that in maybe five visits I've never had to wait, reasonably priced. www.thehottubs.com/
Hot Tubs of Berkeley isn't bad either, but it's a much older and slightly smaller facility, not quite as nice looking but perfectly fine and clean
http://www.hottubsofberkeley.com/ Zen Springs looks fantastic, though it's pretty expensive and I hear they do want ID at the front desk (though you can pay with cash), haven't been there yet
http://www.zensprings.com/ (note: the other places ask that you fill out a registration card when you come in, but you can scribble whatever you want, they don't ask for ID, and they very much don't care)
Drawbacks: No bathroom inside the private room, if you worry a lot about your hair be aware it's a steam/humid environment, and of course you will go through a waiting room
