Florence to Val d'Orcia

If you are not in a hurry you could get there via the Chiantigiana road (SR222) then the SS2 Via Cassia, or even the via Cassia alone. It will take almost 3 hours instead of 1.5 but I think it's worth it. Then on the way back when it's dark, you can return via the freeway.

Not sure if you can do all of this in a day, but if you don't linger in the towns too long, you might:
Sample Itinerary

I would say that taking the A1 misses out on the heart of the scenery you are trying to see. At least, head out by the slower roads, and only plan to return by the faster road if you are running out of time or daylight.

Okay, I think I will follow the route suggested by Roberto, maybe only hit up 2 or 3 towns. I just want to take in the countryside landscape.

Thank you for your help.

I couldn't open Roberto's suggested itinerary, but if you follow his option of taking the S2, you can follow that to the town of San Quirico d'Orcia. From there, you can pick up SP 146 toward Pienza and Montepulciano. SP 146 takes you thru some iconic Tuscan countryside with the cypress trees and beautiful countryside. You will pass through Pienza and can find public parking, and and later can take a turn off for Montepulciano--find a parking lot towards the bottom of the town and either take a shuttle bus or walk up to the town center. From Montepulciano, you can follow signs for the A1 to Firenze and take that back to Florence. It's about a 20 drive to get on the A1 and about an hour and a half more to the Florence airport. It's a toll road so be prepared to pay 3-5 euros when exiting.
We explored this area on a day tour of the Clay Hills (Crete Sensei) with Tours by Roberto (Bechi). He took us to off-the-beaten-path places we never would have seen on our own.

I have some contrarian opinions. IMHO the more south you go in Tuscany, the nicer the landscapes. So I think that driving A1 and then Sinalunga-Torrita-Montepulciano-Pienza exposes you to enough nice landscapes, better than driving the central routes that are long and tiring. The most beautiful landscapes coming to my mind are not even to be seen driving: the back terrace of Caffé Poliziano in Montepulciano and the incredible area just on the back of Pienza. I would prefer spending a little more time in the villages and a little less time driving the endless curves between Florence and Siena.

As an alternative, drive the motorway between Florence and Siena and then the ordinary road to San Quirico, Bagno Vignoni, Pienza and Montepulciano.

Make sure when you are in Pienza to walk along the city wall on what I believe is the south side of town - it is a sort of promenade path from one end of town to the other (it goes by the cemetery). The view of the Val d'Orcia from there is fabulous. Sounds like a great day.

Use the new day trip driving route that was added to the 2015 Rick Steves Florence/Tuscany guide book. It is perfect for seeing the Val d'Orcia.

I think you should absolutely include these in your trip: Pienza, Bagno Vignoni, Montalcino.

Montepulciano is not in the Val d'Orcia--but the Val d'Orcia starts soon after leaving Montepulciano going west.

Iconic Tuscan photo shots are in several places between Pienza and San Quirico d'Orcia in SS 146. (the big yellow villa with cypress trees in front it, the travertine marble chapel (Cappella Vitaleta), the Belvedere). The circular stand of tall cypress trees is on the Via Cassia (SR 2) just north of San Quirico d'Orcia (heading toward Buonconvento/Siena).

Gorgeous views are all along the SS 146, from Pienza, on the road between Castiglione d'Orcia going "the back way" to Montalcino by going through the hamlets of Monte Amiata and Castelnuovo dell'Abate (and stop there to see the monastery of Sant'Antimo and hear the monks chant). From Sant'Antimo, go to Montalcino. On this road you are riding a ridge line and you have vast landscapes on both sides of your car--fantastic.