If you're driving from Siena to return a car in Pisa, you might take the route north up Firenze-Siena Autostrada, and then turn left at Col d'Elsa (perhaps through Monteriggioni), transferring onto SS68 and driving west across the Val d'Elsa to Volterra.
It's a smaller hill town where alabaster is mined/produced. Also has a Roman theatre ruins, as well a couple of interesting museums, one of which is the smallish art museum, the Pinacoteca & Civic Museum Here you can stand in the middle of a medieval palazzo's room and view 3 of the most powerful pieces together (https://volterratur.it/en/poi/art-gallery/).
There is also a renowned Etruscan museum, as Volterra was an important Etruscan center. In fact, the town's origins go back to the 6th century, with it being continuously occupied since the 8th century. You'll find a wonderful mix of Etruscan, Roman and medieval history all wrapped up in one lovely hill-town.
And BTW, we found the food to be excellent with lots of good local choices. See the Comune website: https://www.comune.volterra.pi.it/home
Volterra is so out of the way, it's not usually on many tours. And it has a close neighbor that draws most of the tourists (San Gimignano, which I recommend skipping) and tour buses. The good news it IS on the way to Pisa...sort of...
I've not driven much in Italy, but there is a route out of Volterra that goes north along the western side of the Val d'Elsa, or you can hop over the coastal range and shoot up the autostrade straight into Pisa.
There are so many smaller, less traveled to and visited of which Volterra is one. Since it's on your way anyway... ;-)
