California's Napa and Sonoma valleys are two of the most famous wine-growing regions in the world, and two of my favorite places to visit in the state. In fact, I liked Napa so much that I moved there. The valleys that provide a way of life for thousands of vintners are also the ultimate retreat for wine and food lovers and romantics. Hundreds of wineries are nestled among the vines, and most are open to visitors. Even if you're a teetotaler, the country air, rolling countryside, and world-class restaurants and spas are reasons enough to visit. If you can, plan to spend a couple of days just to get to know one of the valleys. No matter how long you stay, you'll probably never get enough of the romantic, indulgent way of life. It requires stamina, though; eating and drinking to excess can seriously wear you down, unless you're regularly gluttonous.
While Napa and Sonoma are close to each other (about 30 min. apart by car), each is attraction-packed enough that your best bet is to focus on just one of the valleys, especially if your time is limited. I recommend that you read about each below, and then decide which one is right for you -- unless, of course, you're lucky enough to have time to explore both.
Size is the most obvious distinction between the two valleys -- Napa dwarfs Sonoma Valley in population, number of wineries, and sheer tourist volume (and, in summertime, vexing traffic). Napa is definitely the more commercial of the two, with dozens more wineries, spas, and a far superior selection of fine restaurants, hotels, and quintessential Wine Country activities such as hot-air ballooning, set amid mustard flower-covered hills and vast stretches of vineyards. If your goal is to learn about winemaking, world-class wineries such as Sterling and Robert Mondavi offer the most interesting and edifying wine tours in North America, if not in the world. Napa's attractions make it the place to visit for the ultimate Wine Country experience.
Napa Valley is relatively condensed. It's just 35 miles long, which means you can venture from one end to the other in around half an hour (traffic permitting). Conveniently, most of the large wineries -- as well as most of the hotels, shops, and restaurants -- line a single road, Highway 29, which starts at the mouth of the Napa River, near the north end of San Francisco Bay, and continues north to Calistoga and the top of the growing region. Every Napa Valley town and winery can be reached from this main thoroughfare.