High speed trains stop only in Florence and Rome, so if you return the car anywhere else in between, like Arezzo, Chiusi, or Orvieto, you will need to take regionale trains, which aren't as fast.
These are some alternatives to that option.
1 - Return the car in Florence and travel via Frecciarossa to Naples. This option may require some backtracking, because if you visit the Val D'Orcia area in Tuscany, you are already on your way to Rome, and returning to Florence is some significant backtracking.
2 - Drive all the way to Sorrento, return the car in Sorrento, and visit Naples from Sorrento after you are settled at your Sorrento hotel (no need to think what to do with suitcases while visiting the museum). Although from Tuscany to Sorrento is a long 4 hour drive (all freeway), however you can break up the trip and stay in Orvieto and/or other nice places along the way. This option is actually faster and more comfortable when everything else is taken into account (no backtracking, no slower regional trains, no lugging of suitcases aboard trains and ferries, no dealing with driving into city centers of Florence or Naples).
If you choose option 2 above ( that would be my choice) go to www.autoeurope.com or www.kemwel.com (two consolidators now part of the same corporation) and select the best car deal. They work with major companies such as Europcar, Hertz, etc.
The pickup location is Florence. All companies are, besides at the airport, on via Borgo Ognissanti, Hertz also has a location on via Del Sansovino, Europcar is also on via Forlanini.
Your drop off location is Sorrento. Hertz and Europcar offices are across from each other on Corso Italia, near the Circumvesuviana station. Avis is on the same street but further up the road.