We'll be in Tuscany in September and were thinking of renting a car in Siena (after a couple of days in Florence), then drive to San Gimignano to stay in a B&B. We'd then take a day trip to Volterra. Afterwards, we'll drive to Pienza for a five day stay, then go back to Siena to return the car. Would it make more sense to bus to San Gimignano from Florence, bus to Volterra, then bus to Siena where we would rent the car to Pienza? I know a car is the best way to see the area around Pienza, but would it be worthwhile to do the same thing in San Gimignano?
Just take the bus to Siena and spend your time there before renting the car. If you are flying out of Florence, turn the car in at the Florence Airport, as there are not usually drop off fees in Italy.
We especially like Volterra, but it does take a car to get out there because it's off the beaten path--southwest of San Gimignano 30 minutes or so.
The buses line up across the street from the Florence train station--to the right of McDonalds and around the block.