2 weeks itinerary Toscany , CT, Cortona and Orvieto

Is your hotel in a ZTL zone? ( There are lots of horror stories about ZTLs on this forum... be aware so you do not add the tales of woe) Does it have free or paid parking? Often rental car rates by the week are cheaper than the daily rate. Calculate 2 shorter rental periods vs one long one.

I was part of an RS tour when I did Florence, so do not know about long term public parking. Presumably there is some, as the bus had to be somewhere for the days we were there.

The RS book may have some info on parking

There are parking garages in Florence but not all offer daily rates, which is what you will want. Parking is expensive. As another poster mentioned, you will want to compare the price of a weekly rental with parking costs for Florence vs. two separate rentals without parking costs for Florence. Here is the 'official' parking website for a number of secure parking garages in Florence:
http://www.firenzeparcheggi.it/

We are staying outside of Florence and driving in to town on two separate occasions this summer. We have done this before with no problems. Just make sure you know where you're going and where you are parking beforehand. And, we usually have a few parking options/directions as some may be filled.
Happy planning!

Confused why you need the car for the return leg. Not even sure why you need a car at all.
Do you want to visit the countryside in Tuscany at some point?

In Italy with a car rental most of the time it does not cost any extra to drop off in a different city than where you pick up.

Fly into Rome, drive to the spots you want and then drop off the car in Florence before you stay there.
Better to take the train to CT, probably Cortona as well so again the need for a car at all is not clear.
But definitely at the end from Florence to Orvieto is an easy train ride and then train again from Orvieto to Rome at the end.

If you want itinerary help state what you plan on doing/seeing with the car as the places you mentioned are not better with a car.

Thank you very much, everybody.
These are places we want to see:
Florence, Lucca, Pisa, San Gimignano, Volterra, Siena, Montepulciano, Montalcino, Arezzo, Cortona,
CT, Orvieto. To have a car is convenient because we do not want to drug our luggage too much and it will give us more freedom to see countryside.
We really need to do itinerary first, we are just starting, no hotel reservations yet. So - your wisdom will be appreciated.
Thanks

This will give you time to see everything on your list.

Rome>Orvieto>Tuscany>CT>
Florence.

1. Train to Orvieto. 3N.

2. Pick up car. Montepulciano.

3. Montalcino & Pienza.

4. Arezzo & Cortona.
   Stay here. 3 N
   http://www.borgorapale.it

5. Siena.

6. Volterra & San Gimignano.

7. Pisa on way to CT. Drop car in La Spezia. Train to village of choice. 3N

8. CT.

9. CT.

10. Train to Lucca. Day trip.
    Train to Florence. 4N

11. Florence.

12. Florence.

13. Florence.evening train to Rome ?

14. return home.

If you only have 12 nights, do 2 nights in CT.

Sounds like a great trip.