I agree that the valley tends to be very hot during the summer. The temperatures along the coast are much cooler. The wind is fairly consistently a headwind if you head north; however, it usually kicks up in the afternoon. If you were to ride extra early in the day, and camp early, you could avoid most of it.
If you really want to ride in the valley, my suggestion is to take the ferry to Vallejo, then ride the backroads and bike paths to Fairfield and Vacaville, and then head north and northeast, following small backroads as much as possible.