I agree with Pienza. Easy walking, and one of our favorite towns to visit. San Quirico is also easy, and usually not crowded. Another flat town once inside, is Orvieto, although it is officially in Umbria, just off the A-1. It can be seen high on a hill when driving from Rome to Tuscany. All of these flat towns are built high on hills for safety in ancient times, but flat once you are there. For Orvieto, there is a large parking lot at the bottom, and you can take the Funicular to the town. You can also drive up the hill, and follow the P for parking signs, which will put you very close to the town itself. There is a pay box nearby where you pay, and place the receipt in your window. If it expires, however, you could receive a ticket. A great little restaurant in Orvieto is rather hidden, and I do recommend making a reservation. La Palomba.
For San Quirico, park on the back side. There is plenty of parking, and you can enter through the city gates. No climbing involved.