Out of all of the wine counties my favorite is Sonoma County. First I just love the town green. It has ancient trees and is the place where locals and tourists hang out all the time, either tossing the ball, picnicking or chatting with friends.
An historical site to visit is Mission San Francisco Salano which is near the town green. It is one of the oldest buildings and one of the original missions, built in 1823 and is the most northern of the missions in California. Did you know it was named after Francis Solanus, who was known as "Wonder Worker of the New World."?
Second, part of Sonoma County that I really love is Dry Creek Valley. It seems to have missed all the hype that other areas of Sonoma County have. I love the old Dry Creek General Store and just beyond, up in the hills, is probably one my favorite wineries.
It is so unpretentious and they truly love what they do and that it's called Raymond Burr Winery. Above their entrance has a banner, "Wine a bit, you'll feel better". It is secluded up on a hill and the grounds are lovely without being over the top. One of the founders is the lifelong partner of Raymond Burr and his love of the place is palpable.
Also in Sonoma County is Healdsburg, which has some lovely wineries. One with some of the most beautiful grounds is Ferrari-Carano Vineyards & Winery. The plantings and water features are lovely as are the wines.
And don't forget the Armstrong Redwoods State National Reserve. Stunning place to visit.