Are you flying from USA?
If so, are you sure you want to?

I'm flying from the USA with my wife and 3 teenagers; and spending 5 days in Rome,then 7 days in Tuscany near Sienna. I want a car while in Tuscany so that we can do day trips from our villa. Looking at the map, I thought it might be easier and more scenic to rent a car back at or near the airport and drive up the coast for a couple of hours before heading back inland toward Sienna.

Maybe it would be easier (but not quite as scenic)to take a train from Rome to Sienna or Florence and rent a car there?

Thanks in advance for any insight you could give.

You don't say when this trip is, but I'm assuming it's high-season summer. You also don't say what day of the week you contemplate this drive, or how many days you'll have in Siena for exploring Tuscany and what's already on your agenda for those days.

Generally speaking, I would rather train to Orvieto, explore the town for a couple of hours, perhaps have lunch, rent the car (near the train station) and drive to Siena. (Check the open/closed hours of the rental agency before you plot out your day. You may decide not to stay for lunch.) Driving the coast in summer, especially on weekends, could be a slow slog and sadly not as scenic as you might think/hope. Making a return trip to the airport also doesn't appeal to me. Finally, it doesn't give you the opportunity to drive through the Val d'Orcia area (generally along the SR2) which many here consider the prettiest, most scenic part of Tuscany.

HOWEVER, driving at least the first part of the coast offers the chance to stop at Tarquinia, Sutri, Viterbo or Pitigliano. Do a little research to see if any of these interest you more than southern Tuscany.

Thanks, Jean. I greatly appreciate the input. I'll do some research on those coastal cities/villages. We are in Italy during the last 2 weeks of June; and would be making the drive to Tuscany on a Saturday. I have read that the drive along the SR2 is very pretty. . .was just looking for something that would be a little different (scenery-wise) from the rest of our trip.

OK--now I understand.
I have done 3 routes from Rome to Tuscany and the coast road is not worth the time difference--unless you do stop at a site enroute. The Tarqinia idea is a good one, but I also suggest a stop at Civita Bagnoregio and then S-2 north after that---with a brief stop at Bagno Vignoni. I often suggest getting the car in Orvieto, but with 5 of you it may be best to start at FCO.