Ooh, thought of some more.
There are a couple companies that offer bike tours;
Welcome to Napa Valley Bike Tours! has several different kinds, I think. There's one that takes you to different wineries for wine tasting (I don't think the tasting fee is included in the tour price) and for lunch at a restaurant. Napa is so gorgeous, I think it would be wonderful to do a bike tour (I went on one in Paris but haven't done one in Napa).
And if you have some free time to just wander, Yountville is a really cute little town to walk around in for an hour or two; that's where The French Laundry is (
The French Laundry and they're really popular so if you want to visit I'd highly recommend calling beforehand to make a reservation), and there are some vineyards you can wander in, and a nice little main street. It's just off 29, so you can just turn off the road, walk around, then get back on the road and keep driving.
A lot of the higher end wineries are further north, in Calistoga and St. Helena but if you just drive on 29 you'll find tons of places to go tasting starting in Rutherford. If you go alone the Silverado Trail instead (it parallels 29) it's off the beaten track but also has several wineries, so you might get more attention in those wineries if you're interested in really learning about the winery and its wines. NapaValley.com has a map with winery information (
Napa Valley Clickable Map), but I don't think all wineries are listed. If you're a AAA member, you can get a free winery map at AAA offices in California (and possibly everywhere, but I haven't tried outside the state); if you're not a member you can buy one for around $5 - again, it doesn't list all the wineries, just some of them.