The furthest north we made it in CA on our road trip was Calistoga where we spent a night camping. We where in Calistoga for one reason - Indian Springs! We where in need of a little pampering on our vacation and thought that sitting in a volcanic ash mud baths and swimming in an Olympic-sized mineral pool was the way to do it in style!
Indian Springs is a resort that has an amazing spa that I would highly recommend checking out if you're in the neighborhood. I regret that I didn't get any photographs of the mud baths; I was in relaxation mode.
The experience I had was wonderful: after being immersed in volcanic hot mud you will soak in a mineral tub where they serve ice water with cucumbers, than a moment in a eucalyptus and lavender filled steam room, and lastly they wrap you in a soft flannel blanket and you take a nap in a private room (it's amazing!!!) Eric and I then met up at the Olympic-sized mineral pool to eat lunch together and plan our drive to Napa.
PS Another CA spot that sounds amazing!