Thanks for all who gave us suggestions: Here's what we did:
We picked up our Hertz rental car in Bologna and headed for our first overnight stop in Lucca. From there, Volterra to visit the Etruscan Museum (wished we'd had at least an overnight in Volterra) and then arrived late afternoon to our next base near Montepulciano...
San Albino about is 4 km from Montepulciano but it makes a good base for touring the region. We spent 3 nights at I Due Cigni, a very clean and modern hotel for 65 Euro per night which included a great breakfast and a place to park the rental car.
From there we toured Monticchiello, Bagno Vignoni, San Quirico d'Orcia, Pienza, and Cortona. It worked out perfectly as we could have dinner and spend the evenings in Montepulciano.
For touring Umbria, our choice didn't work as well as we picked Perugia as a base for 3 nights but we left after 2 nights, returned the rental car a day early and took the train to Orvieto. Perugia is a congested city--not easy to drive in or find parking plus, the city just wasn't that appealing.
But, from Perugia we drove to Spello (a highlight!) and Bevagna.
We turned in the rental car in Perugia which worked out well as the Hertz rental return is right next to the train station.
Having a GPS was invaluable. Another thing to consider when touring the Hill Towns in this region is that the shops close from about 1 until 3 so plan arrival times accordingly.
We were there in late September and the grapes were heavy on the vines--a really beautiful time to visit!