Favorite Sonoma wineries

There are over 300 wineries in Sonoma County, so one way to narrow it down is to pick a varietal that you love or just pick a geographical region near something else you want to see each day--figure on 4 (5 max) wineries a day, much more will just overwhelm you.  
So for example, if you like zins, Dry Creek Rd(outside Healdsburg) is awesome. The valley itself is about 1 mile long, narrow, and full of small wineries. If you like cabs, Sonoma or Alexander Valley. If you like pinot noir or chardonnay, try the Carneros region (south end of the Sonoma Valley) or Russian River--there are a dozen or so wineries up Olivet Rd, you can combine an Olivet road tasting  with a hike in the redwoods: at River Road turn left and visit Korbel Winery (nice gardens, free tours, great deli) and continue on to Armstrong Redwoods.
All of these areas will offer other varietals, as well, of course, but each has microclimates that work well for a selection of varietals.  
If the weather is nice take a picnic. Most wineries have lovely picnic grounds (the only requirement is if you have wine with your picnic it has to be the wineries--it's not to pressure you to buy; if you have outside wine or beer , it's a liquor law violation for them.) If you plan to picnic in dry Creek valley, pick up your picnic in Healdsburg or where ever you're staying, there's only one grocery store/deli in Dry Creek Valley itself, and it;s usually mobbed. 
You can also narrow it down by kid or pet friendly wineries, historical wineries, etc. 
Post again f you'd like more specific recommendations for a narrower focus. 
Last bits of advice: some of the large wineries offer free tours, otherwise stick with the smaller ones. You'll get more attention, and you'll discover wines you can't get at Costco. And check local events listings or the wineries websites; they may have events while you're here that can be fun (live music, etc.)