I haven't rented a car in Florence before and would like to find an agency that is close to the main train station and easy to drive in and out of the city to and from the Tuscany area. I am planning to stay at a hotel in the town center.
When you check with the rental agencies, they tell you where their rental offices are located. Something to keep in mind for Florence: it is not a fun city to drive in, at any time of the day or night. Parking can be difficult to find (some lots are for residents only, some only open certain hours), and it's not cheap! Our rental had parking so we were lucky. May email your hotel for recommendations. Great city to stay and spend time in!!
Driving in Florence is maddening. But I would arrange for the car from here. I have used a company called Kemwell( you can find it on the internet-- it was the lowest price) They have a number of pickup locations. YOu can check it against a map to see which one is closest to your hotel. We prepaid and it was a turn key arrangement that included insurance requirement. We returned it early and a refund was issued to our credit card when we came home. Make sure you have an international license- we never needed to show it but it is a legal requirement.
If you want one near the train station, look for car rental places on either the Via Borgognissanti or the Via Maso Finiguerra. Both are about a five minute walk from the train station and are easy to find. We rented from a Sixt location there and had no problems.
Go to your auto rentals web site, and check out the various locations that they have, then pick out the one that suits you best. Check out Autoeurope.com for rates and locations.