Central Tuscany Town Recommendations

I think this is a no-brainer. Stay in Siena.

All the Tuscan hill towns are medieval, but Siena was a medieval city, the former capital of a thriving republic that survived 400 years but was decimated by the plague and ultimately defeated by Florence. Siena's golden age was several hundred years earlier than Florence's, so it's a fantastic way to experience the difference between medieval and renaissance Italian cities. (For that reason I generally think it's more interesting to see Siena first and then Florence, but it's fine to do it in reverse.)

The sites in Siena are well worth seeing, and aside from that, Siena is absolutely lovely, especially at night.

Stop in San Gimignano, Volterra, Montalcino, Montepulciano, and Pienza, Stay overnight in Siena!

We stayed near Poggibinsi at a country house, Villa San Giorgio. It’s up on a hill with spectacular views and a decent breakfast. We then ate at Ristorante di Lamole in the village of Lamole which is excellent. Both of these places are up windy roads and take advantage of having a car. We definitely need GPS to find them.

I agree with Lane's comments, but make sure you understand that Siena is a much more urban experience than even Orvieto. That's not a bad thing, but the rest of your interests suggests you might be ... disappointed or surprised. Check the dates that the Siena Cathedral floor is "uncovered" before you book this, just in case it works out for you.

Make sure you can reach your flight home without danger of missing the three-hour check in, before you select your last night stop. Or maybe you are staying in Rome before you fly home? It will be nice to be in Tuscany in shoulder season, but I wonder if some things might be less active, or even closed? Maybe Lonely Planet's book can give some guidance there. Are there any wine festivals?

Leaving Florence, visit both San Gimignano and Siena on your way south. Look at Castelnuovo Berardegna or Montalcino as your overnight stop. CB is less than a half hour from Siena. If you're into wine, without a question, Montalcino.

Either will give you the opportunity to visit some of the Val d'Orcia on your way to Torre Alaina.

We stayed four nights at Borgo Casato in CB and drove into Siena and other towns for the day (when we could pull ourselves away from the view from the terrace dining area ... and the pool.)
Fantastic a la carte dinners as well as wonderful breakfasts.
Looks like the name may have changed to Agriturismo Podere Casato, so I don't know if the dining experience has changed.

gerri, I appreciate your vote for Montalcino. I assure you, we are into wine ;)

Thank you all for your replies so far, I will investigate each of them! It's a rainy day in VT, so today I will relax with my computer and work on some rough plans for our upcoming visit :)