You have a few options to choose from.
You can return the car in Bolzano, and take the train from there.
There are high speed FrecciaRossa trains from Bolzano to Florence so the train journey is only 3h15min. The full fare in Standard class is 58€ per person.
If your Dolomites roaming takes you closer to Verona, you can also take a train from there. In that case the train journey on the FrecciaRossa is only 1h32m, and the full fare is only 47€ pp.
The drive from the above cities to Florence airport (if you plan to return the car there) is at least one hour longer than the train, but in the end it is a wash, since you wouldn’t need to drive to the city center of those cities to return the car and take the train.
The drive is all freeway, Gasoline is about 2€ a liter, so from Bolzano to Florence you might need 50€ in fuel or so (less from Verona), to that you need to add the freeway tolls, which from Bolzano would be about 25€ or so (less from Verona).
So it is really up to you, the level of comfort you have driving on freeways (I presume you are used to driving on freeways in America), the amount of luggage you will have accumulated (train travel and big and/or heavy suitcases don’t go well together), but above all it will depend on what your wife asks (I.e. tells) you to do. She’s not doing the driving, so she will be just as comfortable in the car as in the train. You, on the other hand, need to pay attention to the road.