9 days after Tuscany

What time of year?
What are your interests?
How will you travel- car, train?

Need more info

Yes, please provide more detail--including the existing itinerary. What month? Are you counting time in Rome in the nine days? Will you have a car in Giaole? Umbria would be the obvious choice.

How many nights do you have after Tuscany? Do these nine days include time in Rome? Rome alone needs four nights if you add Vatican City. If you take day trips to Orvieto and Naples, make it six nights in Rome.

While in Florence you can take a day trip to Siena by direct bus (1h 15m) meaning you'll need to add a night in Florence.

We are Traveling this September3-30. We have no plans From the 21 of September till the 30th (we leave on the 30th). We did rent a car for our entire trip. We will be seeing Sienna on a day trip. I have never been to Amalfi or Sorento, but I’m afraid the ride will be too long from Chianti then back to FCO.

We did rent a car for our entire trip. We will be seeing Sienna on a
day trip. I have never been to Amalfi or Sorento, but I’m afraid the
ride will be too long from Chianti then back to FCO

This is really too much driving, in my opinion. No need for a car in Rome, so my suggestion would be to enjoy Chianti & then drop off car in Florence & just take the train to Rome. So now you have 10 days, including arrival and departure? (So that's really 8 full days.) Like other posters, it would really help to know more about what you want to see, is this your first trip to Italy? Ancient history / foodies / just hang out / hurry along and see as much as possible / take lots of food or walking tours? We have spent a week just in Rome twice in recent years and never ran out of things to see, but it depends on what interests you! Someone suggested Umbria, that could be wonderful and worth keeping the car to drive to smaller towns, before dropping the car in Rome.

Please read up on all the ZTL's, international drivers license, insurance etc about driving a rental car in Italy, others here have more recent experiences, some good and some not so good. Driving on the Amalfi Coast is not recommended. IF you're keen to visit the Amalfi Coast, there's a lot written on the Forum about how to do that, but then I'd suggest you take a train from Florence to Rome to Naples and do some work on the logistics from there, it's more work but maybe exactly what you want to do?

OR (and this is what I would suggest) you could focus the remaining 8 days (excluding travel days) in Rome, with some gorgeous day trips to Orvieto (cathedral & tunnels), Tivoli (amazing Baroque gardens and nearby ancient Roman Villa) and / or Ostia Antica (which Rick thinks rivals Pompeii). Seriously, there is SO much to do in & around Rome! We were there last year and saw none of the biggest landmarks, yet had a crammed 8 days with many, quiet (yes quiet) museums visits. Good luck!

We are Traveling this September3-30. We have no plans From the 21 of
September till the 30th (we leave on the 30th). We did rent a car for
our entire trip. We will be seeing Sienna on a day trip. I have never
been to Amalfi or Sorento, but I’m afraid the ride will be too long
from Chianti then back to FCO.

You don't need or want a car in Milan, CT or Florence. You can't drive in the centers and parking is an issue.
All 3 of these locations are much easier via train.

Pick up the car as you leave Florence and have it just for the portion that you actually need a car (Chianti) then return it to Florence and continue on via train.
You can day trip to Siena from Florence by bus.

You haven't indicated if you plan to spend any time at all in Rome? You will need to be staying in Rome or near the airport night before your flight.
With 9 nights I'd consider taking fast train from Florence (after dropping car) to Naples- then on to Sorrento for 5 nights- visit Pompeii, AC, etc. Return to Rome for the final 4 nights.
You absolutely do not want a car in this area.
Availability is going to be very limited at this late date so book something ASAP.

Another option would be to pick a different location in Tuscany- Val D'Orcia area- and spend a few nights there (with car)
Another option is to keep the car and base somewhere in Umbria (with parking!) then return the car in Rome before spending nights there.
Or you could easily stay in Rome the entire time and never run out of things to see and do.

Make sure you understand ZTLs, parking, speed cameras and all drivers must have an IDP.

We already have our IDP and have already booked the car. I have been to Italy a dozen times and this is my husbands third visit. Maybe after Tuscany we can go to one of the seaside isles or coasts? We are foodies and love the blue waters of Sicily and Lampedusa but want to stay on the mainland. Is there a seaside town that we could stay and take a train or bus to Rome for a day? We have been to most important sites in Rome.

From Tuscany you could drive down to the coast below Livorno.
Look at places like San Vincenzo, Baratti and Populonia, Piombino, Campiglia marittima and Massa Marittima, then Castiglione della pescaia, Argentario and porto Santo Stefano, then inland to places like Viterbo, Tarquinia, Lago di Bolsena, Orvieto, Pitigliano.
A lovely time of the year in those places - quiet,warm but not hot, soft pre-autumn landscapes

Visit Umbria, Spoleto, Assisi, Perugia, Gubbio, Spello and Orvieto.