Napa is great fun, but most valley floor wineries will have a good winery setting and expensive tasting. That's OK, it's still great. How many days do you have? Of the "old school" places, Beringer, St. Supery(wife likes) Charles Krug(best bang for $), Mondavi, Far Niente and Sterling get my vote, and Sterling only because of the cable car and view if you are from far away. If you have any favorite wines from Napa, that often makes a good stop too, and see where where your wine comes from! Be warned though, tasting itself is expensive in Napa, and we rarely go there any more. Many places require reservations. Napa has become the Disney Land of wine tasting which is both good and bad. The weekends there are impossible there for me, but I'm spoiled (according to
@mainshipfred) since I've been going to Napa for 35 years. Up the sides of the Napa valley, east and west, are some lesser known, small, super high quality places that are worth exploring. Many you need to call ahead so do your research.
Over the years, one of the ways we have picked wineries to visit, is to get 4 bottles of the same variety and general place of origin, have dinner with friends taking notes of what we like and what we don't, and then visit the winner as a group a few weeks later. That's taken us some places you would not normally go because they are too small or just overlooked.
Sonoma is one step more laid back from Napa. Maybe 2 steps up north. Some familiar favorites there: Jordan, Chateau St Jean, Ferrari-Carano. If you make it up to Ferrari-Carano you'll be in the Dry Creek area which has great smaller wineries to explore. Some other favorites there: Fritz, Dutcher Crossing, Truitt-Hurst (great, great picnic area out by Dry Creek), Armida (another great picnic spot). My all time favorite place in that area is Ridge Lytton Springs. Don't miss that.
And if you have an extra day, slide East 90 miles and up into the Sierra Foot Hills and Amador and Eldorado County.. Laid back, family owned (mostly), great wine. Much less expensive. It reminds me of Sonoma 30 years ago.