You know, you don't need to enter Napa from the South end – you could travel north of San Fran to your heart's content, then cut east, and approach Napa from the west or north. I did this once on rural roads and it was exceptionally nice country. South of Napa you could then go east to Joe's mountain wildflowers, get Tahoe or the Sierra as much as you'd like, and, when all is said and done, race down the 5 through the central valley. (This gets you the northbound direction up the coast very nicely, avoids the desert heat as much as you can, and gives you maximum freedom.)