Wine Country
California is famous for its wine, and Sonoma and Napa are known for their wineries.
I would advise spending more time in Sonoma than Napa.
The cities are very close to each other, but Sonoma feels more authentic and it is less expensive than Napa.

For people who have never been, at wineries you learn how they make their wine and you can also go wine tasting.
However, you may have to pay $15 just to try two different bottles of wine, which is really expensive compared to the wine tasting in Europe, for example (It is free most of the time in France).
Besides the wineries and the wine tasting, this is a wonderful part of California. The landscape of the vineyards is really beautiful, and it is even nicer in the spring when the flowers are in bloom.
While there, you can also visit the Old Faithful Geyser (see Old Faithful Geyser page) or you can go to Calistoga, which is known for its spas with mud baths.
So, this is the ideal area if you would like to spend the day enjoying wine at a winery and relaxing at a spa.