Advice for Renting a Car

Tolls and fuel in Italy add up--very expensive. Many turn their rental cars into Hertz in Orvieto and catch a local train for the just over 1 hour ride into Rome Termini. There you could catch a fast train down to Naples where you can catch a local train to Sorrento. The drive from Siena to Orvieto is on great roads, and it's 125 km--90 minute drive.

Pick up in Siena- drop in Orvieto- Hertz is right at train station there
or drop in Chiusi- more options there but not sure what is avail in Siena- might be just Hertz.

If you need an automatic you might have better chance of that if you went to Florence for your pick up- most smaller locations do not have many if any automatics available

I'd also recommend using www.AutoEurope.com for your rental- they are a US based consolidator- very helpful, can usually get you the best rate.
Be sure to get your IDP- International Driving Permit before you leave the states- can get at AAA for about 15$
and read up on driving in Italy!

Advice for renting a car......

In addition to the International Driver's Permit mentioned earlier (which is compulsory for each driver on the rental form), be sure to do some research on ZTL (limited traffic) zones. Hefty fines for each pass through one! These zones exist in many Italian towns, both large and small. You'll also need to be aware of speed cameras, high fuel costs, fines for driving in bus lanes, etc.

A rental car is the best way to see smaller places in Tuscany that aren't well served by public transit. However, it's prudent to make sure you're aware of the rules so you don't receive an "expensive surprise" in the mail several months after you get home.

If you plan to stay in Siena a few days, then move to a smaller town in Tuscany, you could rent the car in Siena (all major rental companies located not far from the station), go to the location of your choice, then, after your Tuscan fun, drive all the way to Sorrento, where you can return the car. All major companies have an office in Sorrento. The drive from Siena to Sorrento is approximately 5+ hours, mostly freeway.

You don’t need to drive through the city of Naples. You will be staying on the freeway A1 to Naples which then becomes A3 freeway going south. So you bypass the city on the freeway. You will exit the A3 at Castellammare Di Stabia, south of Naples to proceed on regular road to Sorrento for the last few miles. Contrary to popular belief, fuel and tolls actually don’t add up to too much. The tolls would be 29.60€ while fuel could be as low as 40€ if traveling on a diesel car (maybe 50€ with gasoline).

You can of course drop off the car at Chiusi (not far from Siena along the Florence-Rome railway) and proceed by train, but you need to change trains at least once (at Napoli Centrale). Travel time by train is also at least 5 hours, all inclusive. The cost of train from Chiusi to Sorrento is about 50€ per person and up, so for two people it’s a little more than the car, but the train is more relaxing (certainly for the driver).
If you opt for the train option, do not return the car in Siena (too many train changes otherwise), rather return it in Chiusi or further south in Orvieto (Hertz only available in Orvieto).

The road drops down to a main road with one lane each way from Castellammare Di Stabia to Sorrento and becomes busy and can be quite slow.

I agree that the drive around Naples on the A1 is pretty straightforward. There is a pretty decent service area northeast of Naples with the ubiquitous and tasty Autogrill.

In addition to the International Driver's Permit mentioned earlier (which is compulsory for each driver on the rental form)
Blockquote

This "fact" is often stated on this site, but I don't know where it comes from. We've now rented cars in Italy in 2000, 2009, 20017 and 2019 (and other Euro countries at other times) and haven't had the International License ever. Someone told me it was only an issue if you get pulled over by the police, but my husband (who is Italian but only has a Canadian license) just laughed. And he has been pulled over and it wasn't an issue, so . . .

Just curious because I see it stated as gospel and we've never run into it in practice.

As for the being careful where you drive --- yes, big deal! We've slipped up driving into the Vatican (we think) and it cost us about 250 euros (however, the 400 Swiss franc speeding ticket we were expecting never showed up, so you win some you lose some)

@ Nickelini:
That “fact”, often repeated in this forum, is based on the law, namely article 135 of the Italian Vehicle Code.
Since you say your husband speaks Italian, ask him to read it.
Tell him in particular to pay attention to what paragraph no.8 of the same art. 135 says and see if he laughs. Just because a policeman let’s you get away with an infraction, it doesn’t change the fact a certain law is in the books. I was pulled over twice in California for speeding and both times the highway patrol gave me a break and let me go. That doesn’t mean that speeding is now legal in California.

Art. 135 of the Italian Vehicle Code is here.

To add to what Roberto said, it's also often stated that the car rental offices in Italy don't ask to see your IDP. Well, the Hertz office in Palermo did.