What to do with a few hours between Pisa and Volterra?

First I'll give a caveat if you are flying overnight: We learned the hard way that if we are driving on arrival, to make it quick and direct (one stop introduced sleepiness and we would not do it again). So, I would go direct to Volterra, check in and have lunch, then do some of your outdoor Volterra sightseeing that day, and save more intensive driving and sightseeing for the next day.
If you are coming in from Europe, then of course while the day away (but caution you will need to store your luggage in Pisa as that is one place I would never leave anything in a car).

Thank you. We will be overnighting first in London, so we hope to be farily fresh when we arrive in Pisa. Good to know about leaving things in the car in Pisa. Is there somewhere else on the way to Volterra that would be work a stop?

Gotcha, then the field is wide open! I loved tiny little San Miniato, where I stayed the night before a flight out of Pisa. It has many great restaurants and views over the countryside, and you could then do a scenic drive--just head south and meander.
Although it could happen anywhere, I would not feel the same level of concern about leaving my belongings in the trunk there (or any tiny village).
If you are up for a longer excursion, you could drive down the coast, head over to Massa Marittima, maybe the Abbey of San Galgano before looping back up to Volterra. If you are an archaeology/history fan, the park at Baratti/Populonia is one of my faves.

Terrific! Thanks!

I would go straight to Volterra, and maybe stop for lunch at Bocelli’s town (Lajatico) along the way. His family has a restaurant in the family farmhouse. It’s called Officine Bocelli. You will drive by it.

I'd go to Volterra and leave your luggage safely in the hotel and then sightsee from there.