Needing transfer from Cruise Port to Siena

My first question is does the place you're staying in Siena have parking? It's a quite congested city, and parking is sometimes difficult to find. Many stay outside of Siena city center in B&B's and Agriturismos.

I would suggest taking a shuttle service into Rome Termini. From there, you can catch a 70 minute local train up to Orvieto. The Hertz rental office is across the street from Orvieto's train station, and it's a popular place to pickup/dropoff cars.
Orvieto is maybe 1 1/2 hrs. by motorway and 4 lane highways to Siena. It's a very easy drive.
Tuscany is a place that doesn't have train service going to every interesting place. And it's best if seen by rental car.
We especially liked Volterra, San Gimignano and Certaldo--all hill towns. The countryside is very picturesque with rolling hills and small farms growing olives and wine grapes.

You can drop the car at any airport rental location without additional charges.

David,

We are staying at Villa Lecchi. I hadn't thought about the parking there, so I appreciate your mentioning that. Thanks for your other suggestions, they will be most helpful in finalizing the transfer details.

Joyce

Not sure what David is referring when he says take the shuttle service to Termini. I am not aware of any shuttle service to Termini. You have a choice of train service (about three an hour depending on time of day), taxis or a hired car. I would do the train - cheap and easy but require a walk of about a third mile to the station from the cruise port entrance. Nice walk along the beach.