Something that I found interesting were the caves where wine was stored, and disturbed by the 1906 earthquake so they became unsafe to enter. One can get a peek into one (or could some years ago) and see the wine racks and barrels that had to be left and are still there. Something to sigh and wonder about while sampling current production at Beringer Winery, St. Helena (Now owned by Nescafe). Reserve for the Vintage tour: Beringer tours

A tour of the wineries, Sonoma, Napa Valley, etc. etc. justifies a visit to California. The Napa Valley wine train (good food, a fair selection of wines) offers a pleasant and relaxing day, we enjoyed it.
The serious wino, however, will check out the various wineries and plan on spending a week or more investigating those of particular interest to them. Bring a high limit credit card!