Driving through Tuscany HIlls suggestions

Other hilltowns close to Siena and San Gimignano would be Certaldo and the much larger Volterra. FYI: San Gimignano is also best seen after 4:00 when the tour buses leave.
It's all 4 lane and controlled access motorway (with tolls) down to Orvieto--and perhaps a 1 1/2 hour drive. There's plenty to see in the above towns.
If you were staying in Orvieto, I'd suggest you take time to see Civita di Bagnoregio, southwest of Orvieto. Orvieto is a popular place to drop off rental cars prior to taking a short train ride into Rome.

My tip: we turned on our GPS and chose "avoid highways" "avoid tolls" all the way from Orvieto to Termini car rental return in Rome. It was lots of fun because the GPS took us right through the middle of the many walled towns. However it does take two - one to navigate (and reassure the driver lol) and one to drive!

We did not enjoy Civita de Bagnoregio. Very difficult to find parking, and quite a long and steep walk to finally arrive at the town, which had almost no actual inhabitants. It didn't help that it was also the one time a restaurant tried to rip us off (and it was one of Steve's recs!).

But driving around sure is fun!

If you add Volterra, I would make it the first hill town for the day, since there is a large prison that you might need to get around to get back to your car. It's not a security concern, but it's a large facility that they shut down access around in the evening, making it confusing to figure out how to return to your car. And seeing Volterra first that day works well with the suggestion to see San G. after 4pm.

All the towns you listed are worth visiting, but Siena is larger and deserves more time than the rest. I would prefer to spend two nights there so that I could take in the city and spend an evening lounging on Il Campo. Where are you picking up and dropping off the car? Maybe that would help us determine timing better for you. It is truly possible to have an aggressive itinerary of the hill towns (been there; done that), but.... I long to return and spend more time in each. My best memories involve food, of course. Pizza steps away from Alma Domus (my accommodation in Siena) and wine with crostini at the fort in Montalcino.

Siena is difficult to drive right into. It's doable, but.... try to scope out public parking ahead of time and if you feel yourself going uphill steeply, consider pulling over and consulting an actual map, not GPS. In the city, I stayed in Rick Steves-recommended Alma Domus - a monastery with a great view of the Duomo and real cappuccino for breakfast! Outside of Siena, I stayed in Agriturismo Marciano - very convenient to the highways, good breakfast, good dinner if desired (for additional cost, as usual in agriturismos), friendly hosts, and olive grove views.

Orvieto is on a bit of a plateau and I would not drive right up into Orvieto city center. I stayed in Rick Steves-recommended-for-drivers Casa Selita - great coffee, wonderful host, wonderful country house, and walkable to city center. Highly recommend Selita's place!

Just watch the speed zone. Every town that we entered had a speed camera just after the speed limited charged for the town. Don't know if all were operational because some of them looked old and weathered. But we didn't take any chances.

stonecarver, don't overlook that you'll probably be paying to park the car in large towns like Orvieto and Siena. At Siena and certainly at San G, you may have to wait for someone to pull out and free up a space, if you're going in August. And the parking isn't right next to the town square, either. Note that Montalcino and Montepulciano are an hour apart. Because there are so many attractive hilltowns, you might be able to use your time better. Just as an idea (not to make a selection for you, but simply an example of the value of extensive internet research, Monteriggioni and Sant'Angelo in Colle are tiny towns and the first, a bit touristy, but they are close to the main roads and have trivial parking problems. You could visit both of them in two hours, if you rush. You might regret how small those towns are, but it's nice not to be fighting hundreds of your closest personal friends for a seat at a cafe table ... ....

When you say "easy drive out of Siena and Orvieto", you need to say whether you have hotel parking of some sort. Both those massively touristed (but very attractive) towns have giant parking facilities outside the walls. You won't, most likely, be "inside" the town when you depart. Have you read about ZTLs?

Joan, you didn't mention ZTL signs inside the walled towns. Wasn't that an issue? When we arrived beside Siena, one of my problems was that the GPS wanted to take us THROUGH the CITY to get to the outside the wall parking I had read about. That was a ZTL no-no!

SR222 ...nuff said