If you think wine tasting in the Napa Valley is all fine and good, but you could really go for something a little more hands-on, I've found just the thing for you. Copia, the wine and food (and arts, and gardening) education center a few blocks from downtown Napa has started up twice daily weekend wine blending workshops. At the end of the class, you get your bottle of wine. Some wineries also offer wine blending workshops, but I don't know of any that offer them so casually and regularly.
Other wine classes available (you can call ahead or get tickets at the admission counter when you get there) include Winetasting 101 (a good way to hone your method before hitting the wine-tasting trail), a daily wine-tasting class, and an introduction to wine and food pairing. I like these because the classes are fairly affordable, don't require a whole lot of advance planning, and they add a new dimension to a wine-country trip.