The stretch between Rome and Florence is just lovely with many wonderful stopping places if you have a car. We have rented at the Rome airport (or elsewhere in Rome) several times and dropped in Florence at the airport. If it is only for transportation from Rome to Florence, I'd take the train which takes 90 minutes, but if you have a few days then a car is great.
There are many Etruscan tomb sites north of Rome. Obvious are Tarquinia and Cerveteri with huge complexes, in the hills around Sovana there are tombs and you can get a map in Sovana to drive to them. Saturnia with a simply enchanting waterfall of hot sulphury water in a public park is also worth a stop. They don't have changing facilities so come with suit on under your clothes and plan a quick change by the car afterwards. This is one of those most amazingly beautiful things we ever stumbled onto.
The area of Tuscany south of FLorence is filled with lovely towns and abbeys and such.
The last time we made this trip -- pick up at FCO and drop at Florence airport we spent two week in Montepulciano doing day trips into the Tuscan countryside each day.
Drop at the Florence airport and take a cab for 20 Euro into town -- or get a bus. The odds of encountering an expensive ticket trying to drive into the train station car agencies are very high. It is common to receive fees of 30=50 Euro from the car company just for the onerous task of providing your info to the police and then months later, you are likely to receive a traffic ticket for 100 Euros or more. And if you get a bit confused and have to go around a block you may get two. Driving in florence is not worth the risk. They really don't want people doing it and they enforce it strictly.
