|So many choices!|
How long is your visit?
In a full day you can reasonably expect to go to about four wineries. Much more than that and you risk intoxication with a lesser issue being you can't really discern what you like any more. That is, you're wasting your time and money unless getting buzzed is the goal in which case your hotel's bar is a better option.
In one day you should only visit one area, such as either Sonoma Valley, Russian River Vly, Dry Creek Vly, or Alexander Vly.
If visiting for several days expect by about the third day you and your palate will want a break from wine tasting. So if you've done two full days of winery visits maybe plan on only one in the afternoon of the third day. In Sonoma County there are lots of alternative activities available--ocean to mountains to museums to shopping.
Any particular varietals you are looking for?
Old favorites or something new? If you mostly drink Cabernet at home are you looking for more great Cabs to stock up your cellar or do you want to explore other varieties? Maybe you didn't realize Sonoma County is full of great Sauvignon Blanc and Zinfandel. Consider wine tasting as a learning experience. When will you have another chance to try some new things?
A generalization, but if you're looking for Chardonnay or Pinot Noir concentrate on Russian River Valley, Dry Creek Valley for Zinfandel, and Alexander Valley for Cabernet.
Big wineries or small?
Do you want to taste stuff you might find back home or are you looking for wines that aren't available to you except in the tasting room? The smallest wineries tend to be more rustic with tasting in a small, chilly cellar vs. in a 5,000 sq ft tiled glass-windowed high-ceilinged room (most tasting rooms are somewhere in between). There's a big difference in the experience at David Coffaro Winery (about 5,000 cases of wine produced a year) vs. Chateau St. Jean (400,000 cases), for instance.
The wineries that are the most convenient to get to and are the most well-known will be the most crowded.
Where are you staying?
What area do you want to stay in? Sonoma County is larger than the state of Rhode Island. There is only one main road (US 101) with the rest having more leisurely travel so don't try to crisscross the region in a single day. If you have a particular area in mind then stay close by. For instance, if you plan on spending most of your time in Dry Creek Valley then stay in Healdsburg, not Sonoma. Santa Rosa is centrally located to all of Sonoma County and even the northern half of Napa Valley.
Any particular amenities?
Are you looking for food, kid-friendly, dog-friendly, other stuff to do besides taste wine, views, or tours? You'll have to do a bit of homework ahead of time to see who offers anything special that's of interest, but here are some ideas:
Food: Coppola Winery for a sit-down meal. Korbel for a nice deli.
Kids: Most places are okay, but the kids won't last more than one or two wineries a day. Best choice is probably Francis Ford Coppola (swimming pool, movie museum, restaurant).
Dogs: Most smaller wineries are okay, but it's best to check before rather than be turned away.
Views: Paradise Ridge, Sbragia
Tours: Korbel, Benziger, Simi
Art: Imagery, Paradise Ridge
Gardens: Chateau St. Jean, Ferrari-Carano
A lot of people don't plan much. They just get a place to stay then ask the front desk where they should go wine tasting. At that first winery they ask for more suggestions. Maybe they just see an interesting looking place and decide to stop. "Look, there's a fire truck out front" or "the sign says they got eight gold medals."
Most wineries you see while driving through the vineyards will be open to the public usually opening sometime in the late morning and closing late afternoon.
There are all kinds of experiences waiting for you.
Disclaimer: I work at one of the wineries mentioned here