Staying central to Florence or Siena

Florence is a big city, if you stay there you don't need a car. Same with Siena, in town is best not to have a car. The two are connected by frequent bus service (or train, although the bus is faster in this case). The other smaller towns in Tuscany are best visited by car. Also if you want to stay in an agriturismo, you have no choice, you need a car. Agriturismo are farm houses converted into vacation homes/hotels. They are often in the middle of the countryside with little or no public transportation within walking distance. You can certainly use an agriturismo anywhere in the Florence or Siena provinces and make day trips with your rental car from there. Civita is not in Tuscany. It's further south in the province of Viterbo (Lazio region), west of Orvieto. It's a little further away but it can be done by car from Tuscany as a day trip. With public transportation it would be a little harder in a day because Civita is a little isolated, rather than on a main rail line, and getting there would require a combination of trains and buses. Given the places you want to visit I would split my stay in part in Florence without a car and from there visit Siena by bus (Siena is not car friendly either). Then I would rent a car and find a place in the countryside near Siena for the rest of the period. From that country location (agriturismo?), I would visit by car, the small towns that you mentioned (and even more), which are more suited to be visited by car. This is a good guide on the many Tuscan towns and villages, including the ones you mentioned (Civita is not featured, since it's not in Tuscany): http://www.borghiditoscana.net/eng/index.html

Thanks Roberto!