Hi Gretchen,
I don't have big advice for Sonoma as we haven't spent much time there, but we go to Napa a lot and just got back from a quick weekend trip. If you don't know where you're staying yet, search no more.
The Chanrick Inn in Calistoga is the most wonderful accommodation. It is beautiful and Channing and Ric (owners) are wonderful people. It's not your typical frilly B&B. Ric is an amazing chef, so you're not getting toast and eggs for breakfast.

It was such a treat for us.
2 new wineries in Napa on my MUST visit list are:
Joseph Phelps (You need a reservation, so make sure you make one. Great place and great wines.)
Vincent Arroyo (Silverado trail near Calistoga) - We especially liked this one because this man took our group into the back and we did barrel tasting. We asked his name and he said, "My name's on the glass." It was a wonderful experience talking to the winemaker about his wines. The wines were pretty outstanding, as well.
Food - If you like mushrooms,
The Martini House has mushroom tasting menu that is out of this world. Even if you don't, it's worth checking out. The chef's passion, apparently, is mushrooms, though.
Have fun and RELAX!