Hello, we’re planning a trip to Tuscany next fall & I'm very indecisive about renting a car while staying in Siena (the stress of driving) to travel the “Heart of Tuscany” drive or better to book tour guides or day trips to explore Tuscany.
One of the big problems you may encounter are ZTL zones where you can not legally drive. Doing so will result in large fines and admin. cost. Another problem is speed cameras. But perhaps the most important item to consider is the driver. Everyone else will enjoy the the beautiful views; but not the driver. He / she will have to focus on the roads and traffic. Don't mean to be Debbie Downer, just offering food for thought.
If it’s only for one day trip for one day, it is best to use a private tour, maybe with a small group, because the logistics of picking up and returning the car, are not worth it for just one (not even full) day deal. Prices for a small group (up to 8 people) with a van, for half day (5-6 hours), vary but start at about 100€ per person, depending on tour.
If you plan multiple trips while in Siena (or Siena area), then the cost savings of renting your own car make it more logical to rent your own car for a few days for those day trips.
If you choose the latter option and you intend to stay in the city of Siena itself, be aware of the traffic restrictions in the city center, therefore you need to select accommodations accessible by car and preferably with parking arrangements.
This is my 2 euro cents.
Rent a car in Italy=$$$ in tickets