I think with the amount of time you have, trying to go up past San Fransisco into the Redwoods would make for a bunch of driving - doable but probably not as much fun than if you had gone later in the year around spring time.
I'm going to agree with with 70sixter about the route to take and places to see. We're getting a lot of rain right now as I'm typing but who knows...the days you plan to be here could be beautiful. (Here being San Diego.)
I drive up and down the San Diego coast quite often in my '01 Box or the '60 MGA. I'm currently planning the route for the upcoming San Diego Rolling British Car Day and can recommend plenty of great coastal and back country roads with lots of twists and turns.
I also went up to Napa in the Box and then back down Hwy 1 recently and it's a great drive but it's a LOT of driving.
Feel free to send me a private message and I can help you plan a route. And congrats on the new Box!
