Tuscany - less traveled towns

Easy. Pick one you have never heard of!

Tuscany does have that "throw a dart" quality where it is hard to not find a pretty town. You might narrow down by identifying what you want to see and do during the day, or share some particular interests. I found the northern mountainous bit (Garfagnana) had a get away from it all quality. I've probably gone on ad nauseum about how much I love the coastal south (Maremma).
These two websites are both super helpful:
https://www.visittuscany.com/en/index.html
https://www.discovertuscany.com/
Also, where are you before and after, and how many days do you have? That might spark the right suggestion for us.

I enjoyed Arezzo - not unvisited by tourists, but definitely not on most Tuscan itineraries. Or you could look at Pienza, a bit less touristed than the biggies.

I do agree that you could just look at a map of Tuscany and then google some of the towns you see that you haven't heard of to see if they have anything of interest to you. That's how I find a lot of places to visit when I don't want to follow the RS (or other travel guides) suggestions and get off on my own.

Our base will be in Siena and we will just look on a map and drive to some of the smaller towns in the area. Most of the hilltop towns look the same, but still intriguing.

TRIP 1 (to the north west): Colle Val D'Elsa, Monteriggioni, maybe even Volterra if you have time and like fast pace.
TRIP 2 (to the north-northwest): San Gimignano, Barberino Val D'Elsa, Certaldo
TRIP 3 (to the south): Montepulciano, Pienza, Monticchiello (maybe add Montalcino if you have time).
TRIP 4 (to the east): Lucignano, Cortona, maybe even Arezzo if you like fast paced, although it would be quite a bit in a day.
TRIP 5 (to the north): Chianti towns (Panzano, Castellina, Greve).

1 and 2 are in the general area (if one excludes Volterra, which is a bit farther to the west)

To parphrase Horace Greeley "go South young man" : Pitigliano, Saturnia, Sorano, Sovana, all "less traveled". Also a fan of Maremma.

Check the Tuscan section of the website of the Borghi Piu Belli d'Italia (the Most Beautiful Villages of Italy). The website has a write-up and a gallery of photos for each village (in Italian). https://borghipiubelliditalia.it/toscana/
The towns vary in size and level of services, some would not be a good choice for overnight.

Last September, we spent several nights each in Tarquinia, Pitigliano, Massa Marittima, and Volterra --- all were lovely and interesting and pleasantly busy with Italian and other European tourists. Then, six nights in Pisa, which had plenty for us to see and do besides the tower. Not what you want, though.

Re-visited Tuscania (which might fit your requirements), and visited San Gimignano (NOT "less traveled" and we left as soon as we could after seeing some frescoes I needed to see), Barga, and a few other places.

Our favorite town that was still in Tuscany before we drove to Liguria is called Pontremoli, and we sooooo wished we'd known how nice it was and had planned for more than just three nights. I don't know if it's too far off the beaten path for you, or doesn't have the classic Tuscany scenery, but we sure liked it --- great places to eat, cool museum in a castle, bridges, hills, interesting history, and medieval streets.

Interesting that Pontremoli is mentioned, it's been on my list!
I'm flying into Pisa,only because it's 34 minutes to Lucca, my base for the first 8 days of my trip in May. Pontremoli is about 1 hour 40 minutes from Lucca by train.Decided to spend the night, very reasonable accommodation cost to enjoy the river and walking in the afternoon and evening.
Another trip from Lucca is Barga ,about an hour away from Lucca by train. Beautiful small mountain town. A bus goes from the train station to the town. Unusual connection with Scotland,apparently a 2 way migration has been going on for a long time.
Also on my itinerary Castelnuovo di Garfagnana, castle with a view,interesting town and less than an hour by train from Lucca.

Forgot to mention Terme di Montecatini. Only 34 minutes by train from Lucca.
Ride the small funicular up to the top. Nice town to stroll through looking at the spas architecture. Nice for an afternoon.

Here's a good blog by a woman who lives in Lucca and takes good photos --- there's at least two posts about her visit to Pontremoli (where she went on my recommendation): https://www.twopartsitaly.com/

She's visited and blogged about other small towns in Tuscany and elsewhere in Italy.

The Lunigiana region of Tuscany is a treat, though not a secret. Pontremoli is one treat, as are the nearby towns of Bagnone, Filetto, and Filattiera. https://www.apathtolunch.com/2014/05/exploring-pontremoli-ten-things-to-do.html