I'm looking to make a day trip to Napa while I'm visiting San Francisco. What do you guys think is the best way to explore all of what Napa has to offer?
If it truly is a day-trip requirement and wine tasting is the purpose, then I'd look into a tour that provides transportation, though that will restrict you to only the wineries they choose to include.
If it is possible to make it an overnight, that would definitely give you much more flexibility and options for a self-guided tour, including a nice dinner in the region as well.
Unless you have specific wineries you want to visit, my favorite way to visit Napa Valley is to have one set first stop in mind (or just pick at random) and then spend the rest of the day hopping from winery to winery based on the suggestions of the staff in the tasting rooms.
I would recommend at lest one night stay. Drive up early, use Platypus tours, grab dinner in Yountville, and head back the next day.
If your purpose is wine tasting, I would definitely recommend hiring a driver for the day. They can be your guide as well as your driver and find out what you have in mind, what kinds of wines you like, what you want your overall experience to be and gear your day to that. Find a good wine-centric travel agent to make recommendations for you.