Car route options Rome to Siena

I consider Orvieto to be a very worthwhile destination, that can be done as a stop for a few (or, several) hours. It would be a shame to miss it entirely. I confess that I have not seen the Italian seashore (except Sardinia!), but I don't understand why so many Americans who regularly visit the Atlantic and Pacific seashores prioritize it. (I'm not talking about CT or AC architecture, I'm talking about beaches in the sunshine.)

I did find the public beach near the Capri port, and the lakeside beach in Castiglione del Lago to be of brief local civic life interest, but neither warranted hours of driving.

We drove to the Rome airport from Sinalunga, using toll highways, at the end of our Tuscany trip. There was frequent construction, narrower lanes than we have in the US, and heavy truck traffic. The views were OK, but nothing special. If you are going to have the car for several days, I would concentrate on getting there, and plan on leisurely drives on curving, sloping local roads as part of each day's excursions. Be sure you have read up on insurance, ZTL, parking, and IDPs.

I agree that a car is extremely useful in Tuscany. We got off a cruise in Civitavecchia, which was a reason to rent the car there, rather than train-ing back to Rome for a long-distance train. We stopped in Viterbo and Orvieto, getting to Sinalunga in time for a light dinner. But you do have the option of taking the train from Rome to Florence, and enjoying the view out the windows.

Forget about the seashore route: unremarkable scenery (less nice than the inland motorway), no sea views, lower road quality. Going via Orvieto, and stopping there, is indeed the best call!
But in Italy, make sure to leave NOTHING visible inside a parked car. No bags, no luggage, not even clothing. Car break-ins do happen...
So make sure get a car with a luggage cover (most have one, but check before you leave!).

Thank you Tim and Balso. We much appreciate the input!

The SR2 is a very scenic road through Tuscany, which you could pick up a little over an hour out of Rome. The entire trip from Rome to Siena using the SR2 would take about 3 1/2 hours. SR2 is not the Autostrada, but you can make good time on that road while still seeing some beautiful Tuscan scenery. If you have time, Bagno Vignoni, San Quirico d'Orcia, and Buonconvento are worth a visit when traveling SR2, and Montalcino and Orvieto aren't too far off SR2.

We just drove back from Siena to Rome on a Sunday in August and the reverse traffic (going from Rome to Sienna) looked horrible, massive backups the whole way. I guess my point is the "scenic" route may be just as fast if you're going on a busy day where the main route backs up. Without traffic, the fast route was pretty and pleasant.

Personally I'd say just get to Tuscany - it's pretty everywhere, the real magic I found was enjoying wherever we were, not worrying about trying to stuff a lot in. You really can't go wrong!

Nnorell you could Google about the Maremma and see if that's what you are looking for. Otherwise the Elba island is quite famous.

Siena to Rome on a Sunday in August and the reverse traffic (going from Rome to Sienna) looked horrible

Precisely, on a Sunday in August when everyone is going back north. Either the OP is doing the same in 2022 (I hope not) or he will hardly face a similar situation.

Luggage drop in Orvieto:

https://www.orvietoviva.com/en/orvieto-train-station

Going up to town take Bus 1. to the Piazza della Repubblica [tickets from the shop in the station]. The funicular goes to Piazza Cahen, not all the way up; is handy for walking down the hill and returning to the station from there.

If you are ok going slow (will be an all day drive probably) and want to see some Etruscan sites before you get there, go up the coast and hit Cerveteri and/or Tarquinia. I got non the coast road a bit north of Tarquinia going south so not sure how it is from there up to Siena, but I thought Etruscan sites were interesting. Parking was fine near both sites although the town of Tarquinia is a little busier (the painted tombs are just outside the center), and I had luggage in the trunk. Maybe got lucky but it was worth the risk.

thanks everyone for the advice. Much appreciated.

Take the Via Cassia (SR 2). It is the ancient road connecting Rome to Florence (passing via Siena) before the era of freeways. The prettiest historic towns are more or less along that route.