Trip to San Gimignano & Volterra from Siena

I would not advise using a third party vendor, such as Get Your Guide. Work directly with the bus company, there are dozens.
Driving yourself can involve parking issues, and long walks from the parking areas. It's a hard call between driving yourself and using a tour. Tuscany is huge. Riding all day in a van/bus also has issues, and bumps and hairpin curves, and is a long day getting back to Siena. Driving yourself and wine tasting has obvious limitations.
It's a tough decision. Good luck !

I did a good bit of driving in Italy when we stayed in Cortona for three months. We were in Volterra, Siena and San Gimignano (several times for the last two). I do NOT remember hairpin turns, at least none that were a problem. I think that ride is very doable. But, as you may know, Italian drivers speed! I just tried to ignore them. One interesting note, I received TWO different speeding tickets one year after we returned home!
Parking - in Volterra we were at a hotel so not sure about public parking options. Both Siena and San Gimignano had parking that worked out well with just a short walk into the main areas.
As for hairpin turns, there are a number going up to Cortona, and certainly the turns on the Amalfi coast are difficult. But I think your route should be fine. HOPEFULLY someone more local to the area will weigh in.

Forget renting a car, parking is a problem in those cities.

Also, many tourists that rent cars in Italy get pricey tickets months after they return home.

Parking is at a premium in these locations. I would opt for the day tour with a local company.

September 20th 2022 we had no problem at all driving to or parking at either town. San Gimignano, where we went to see some frescoes rather than the town itself, was full of busloads of other daytrippers. But if you just need a car for one or two days, it would save you some time and trouble to find a tour. Plus, whoever would be the driver if you rent a car could look at the scenery more on a tour.

Nancy - that is a good point. I did the driving in Tuscany.....and you do have to pay attention. But we liked the freedom of a car and stopping at other little towns.

You could use public transportation (bus) to reach both from Siena. From Siena to San Gimignano you would take bus 130. The trip should take about an hour. Going to Volterra you would take bus 130 or 131 "Ordinary" with a bus change in Colle V. Else to bus 770. This trip would take just under two hours. The website for the bus is https://www.at-bus.it/en.

There are absolutely no problems parking in either Volterra or San Gimignano.

In Volterra park at the parking structure below:
https://maps.app.goo.gl/TKVV63BFUEqToYse7?g_st=ic

In San Gimignano park at the lot below:
https://maps.app.goo.gl/sg5tEDzqmqEiNRRU8?g_st=ic

Save the locations in your GoogleMaps app.

If you go to SG and Volterra on the same day, the road connecting the two is curvy but you don’t need to be Lewis Hamilton to be able to negotiate those curves. Just don’t speed.

San Gimignano can be visited easily by bus from Siena, taking the frequent #130 bus from Via Tozzi bus terminal in Siena.
Going to Volterra by bus is more convoluted as you need to change buses at Colle Val D’Elsa.
If you want to hit both towns on the same day, the rental car is the way to go. But if you have multiple days in Siena you might even consider renting for multiple days and do a few more days trips. Siena needs a day, if you have more days you can rent a car for those days.

You won’t get any tickets if you park where I said and avoid speeding.