If you are only planning on doing wine region touring and tasting for one full day, my strongest possible recommendation is to spend it in Sonoma county rather than the Napa Valley - in the Russian River and Alexander Valley areas. Napa has become an overpriced version of Disneyland relative to most of Sonoma, with costly tasting fees and canned tourist experiences being the norm at the overwhelming majority of wineries. Surprisingly, Sonoma has mostly not suffered from this same affliction. In terms of the wines and wineries themselves, Sonoma is of similar quality to Napa, but the wines made there are more diverse, reflecting a broader range of grape varietals and microclimates than are found in Napa. Unless you are specifically looking to learn about Cabernet Sauvignon and Cabernet-blends, which dominate Napa, Sonoma is absolutely the better choice for most tourists.

I also STRONGLY recommend doing the wine touring on a weekday, rather than a weekend, as the wineries are far more crowded on weekends in both the Napa and Sonoma regions and you will get much more personal attention from the wineries' hospitality staffs on a weekday.