Apart from Firenze and Siena, Tuscany offers several other visit-worthy places, which you could combine in one trip.
One of them is Montepulciano, on the border between Toscana and Umbria. It offers a good view of the landscape while being surrounded by picturesque beauty.
Then you could move further and drive through the glowing, rural landscapes of Val d’Orcia towards the famous San Gimignano. This is a medieval village surrounded by both old city walls and enriching panoramas.
And last but not least, you could visit Lucca, also one of the greatest and therefore most popular places in the whole of Toscana, with magnificent buildings and unique structural designs on, for example, the churches, like the San Michelle in Foro.
The distance to those places, of course, differs, but having a car definitely gives you the optimal experience, cruising through the typical landscapes.