Which Hill Towns to Visit?

San Gimignano is a very popular choice in the vicinity, though it tends to be thronged with tourists. Lucca is another good possibility. Pienza would make a nice stop on your way north from Orvieto; also Montepulciano or Montalcino. I'm also not so in love with Siena.

I'll also second Pienza. It's a flat (very rare in a hill town!) and has some of the best gelato I've ever had. I was there just over a week ago and went to Buon Gusto after coming across a Rick Steves Facebook video about it. Lovely owner and the gelato is very fresh and has the best flavours (though only 8 to chose from). On the day I was there, there was no coffee flavour because the cafe owner nearby had taken the day off ...

Montepulciano is another favourite. On this trip we stayed at an agriturismo just outside. Also stop by the large church just below the walls if you have a spare 10 - 15 minutes.

One of my favorite multi day drives. Castiglione del Largo>Montepulciano>

Montalcino>Pienza>Castellina in Chianti>Volterra.

Also not a great fan of Siena...2 days was enough for me.

Thank you all. Will check out all your suggestions. Had thought of Pienza and Montepulciano. Will look at the other ideas as well!

I would take advantage of the car to see towns that are not served by train. Montepulciano, Pienza, and Montalcino fit that description; Volterra's not far from the train line but it's a great destination; Massa Maritima is a possible stop on the drive between those (not in Rick's book). Lucca looks like a nice place to live, but I don't think it has as much charm as the hill towns. Lucca, Pisa, and Siena are all easy to reach by public transport (i.e, on another trip).

Thank you Laura! Have been busy reading about and planning visits to Montepulciano, Pienza, and Montalcino. Will have a look at Massa Marittima next. We are staying near Volterra, so all look doable.

Todi is not far from Orvieto and is beautiful. Further afield is Gubbio, one of the most medieval of the hill towns. I also love Lucca.

We did a tour and visited Montepulciano and Cortona and absolutely loved both of them - great vineyards in Montepulciano and a fun little castle to climb to the top of. Cortona had really beautiful views and very narrow, fun little roads to walk through.

Day 1 - From Orvieto to Montalcino via Montepulciano
Day 2 - From Montalcino to Volterra via San Gimignano
Day 3 - Volterra to Florence

My family and I loved San Gimignano. My husband hates crowds and we tend to stay away from crowded tourist places.... but, we still loved san Gimignano. We did a short drive to Volterra and to be honest was disappointed. Ended up only spending a couple of hours and left.

Thanks everyone for all the suggestions! Appreciate it, am now happily planning those days.

Siena makes more sense as a day trip by bus from Florence. You can't use your car in Siena so it would be a waste of a rental day.
Although Rick Steves has popularized Montepulciano to North Americans, that is not the top hill town destination in Tuscany.
San Gimignano, Volterra, Pienza, Monteriggioni, and the Chianti Hills are by far the top destinations in that part of Tuscany. They tend to be more crowded on weekends, but they aren't worse than Orvieto.
If you want to add others to the above, you can certainly do so. Buonconvento, Montepulciano, Montalcino, Cortona, to name a few, are also very pretty. Cortona is also another top destination, albeit more to the East in the Province of Arezzo.
The itinerary should be in this order:
Orvieto, (Cortona), Montepulciano, Pienza, Montalcino, Buonconvento, Monteriggioni, Volterra, San Gimignano, Chianti Hills (route SR 222), Florence.
Plot it on Google Maps to see it.
Not sure you have time for all, but it's up to you. It depends on how long you stay in each.
If you choose a base (a hub) from where to visit all, then near Monteriggioni is the most central.