Hi there--Dave gave a great answer to start--there are many options in Sonoma and Napa counties. I just spent the better part of last week with buyers from Provence who were asking the same question. It is entirely a function of your tastes for terrain, climate, architecture and of course, what you are willing to pay.
What I love about living here is that whatever area you choose you have easy access to the whole gamut of communities, most of whom have truly fine restaurants, their own farm markets, shops and wine tasting. Plus the country roads that you drive from one to the other (or bicycle) offer fabulous views of vineyards, redwood forests, apple orchards, coastal and river views. As a former out of town buyer myself, I have enjoyed introducing my clients to the wine country of Northern California--generally you will have a sense (as will I) of your preferences as you deepen your search for the right second home. With out of town buyers, we review a selection of homes that meet their overall criteria on-line before their visit, and then view a representative sample during your actual time here. It helps to allow time of course to kick back and find the place that feels right to you.
Generally, Sonoma, Healdsburg, Kenwood and Glen Ellen are on the pricier side. Even in this market, some of the most desireable properties are still selling quickly because there still is a decent market for these types of properties. That said, prices have definitely come down in the past couple of years, but not as far as the overall county median has dropped as the second home market is a different one. Good luck with your search!