What is the best place for a sunny car camping spot near a swimming hole within a couple of hours of San Francisco?

I want to do a 1 night trip for my bday coming up and go somewhere that will be sunny and warm. I’m thinking of staying away from spots in the forest that will be all shady and cold for that reason, which is why I am not so into Big Sur or Big Basin. I was considering the Yuba river, maybe up near Pt. Reyes… any other great places in the greater bay area? I will likely need a site that will accommodate a large group, perhaps 20 people? Also: I don’t want it to be a place overrun by families or kids….