Hi Robert – sounds like you have a great drive through Chianti planned. If I had to pick just 1 or 2 towns to visit, I'd probably pick Greve and Radda to visit.
For Greve, the town itself is a busy wine hub and there's much to see, good restaurants in and nearby Greve, and lengthy list of things to do. The center of the Greve action is at Piazza Matteotti, and you'll find nearby the tasty Antica Macelleria Falorni (if you like salumi), some great wine shops and artisan goods shops. If you're into history and architecture, you can check out Santa Croce church, the Museum of sacred art (Museo di Arte Sacra di San Francesco), and nearby castle of Montefioralle. In Greve, it's interesting to hit the Wine Museum (Museo del vino) to get a sense of the history in the area (and a great winescrew collection), however I believe it is closed at the moment for restoration. You can check here.
Radda is smaller and more medieval, and drenched with winemaking history. It's nice to simply cruise the central town area. I'd also recommend you check out the oldest winery in Italy (from the 11th century), owned by the Ricasoli family, at Castello di Brolio. If you want to taste some local wines, buy a few bottles, and generally enjoy an enoteca atmosphere, I'd recommend you check out La Bottega di Giovannino in the center of Radda at Via Roma #6 (website here). Nearby Castello di Volpaia is also charming and a great place to sample Volpaia wines.
Regarding wineries to visit, the list is endless. I'd recommend you check out these (click the name for more detailed info):
If you can make the time work, don't miss lunch or dinner at Osteria Le Panzanelle.