Viansa - Great tuscan style place, tuscan grill, shop with gourmet items (and tastings, yum), really good wine, picnic benches outside where you can eat your tuscan food and look over the vineyards. Its just before the Napa Sonoma Split on the right hand side (if you are headed north, it's a little easy to miss, it's the same turn as the Sonoma County Visitor's bureau, but to the right)
Clos Pegas - The wine is Okay, the art is great, the owner has an extensive art collection to view, there are great wine caves that you can see. The art alone is worth the stop.
I think it St. Supry that has the learning center, where you go through the process of wine making (though tours at all of them cover this stuff, this one is more interactive), I think it's St. Supry it's been a while.
Have a great time on your trip, it's a beautiful area, and the heat's finally broken (whew)