Retuning to Rome one day to spend close by.

I enjoyed Orvieto a lot - easy train ride to Rome.

Will you have already visited Rome?
Are you flying out of Rome, if so—what time? You will most likely need/ want to be in Rome night before flight

Where will you have already visited? Knowing that would make suggestions easier.
Orvieto, Assisi, another Tuscany town could all work.

Do you have just and extra “day” or is it an extra “ night”?

We will have spent 4 days in Rome ,then on to Montisi area for a week stopping in Orvieto on the way up and visiting multiple towns around during our stay. Siena, Cortona, Assisi, San Gimignano, Montalcino, Montepulciano, Saturnia. Val d’Orcia.
When heading back to Rome we have two days, will want to spend the nite in Rome before we fly out the next A.M. and this leaves 1 day for? Was looking at Porto Ercole however rooms are pricey August 29th, 2019 for 2 rooms.

Will you have seen Florence?

Have you considered going straight to Orvieto for a night or 2 upon arrival and moving all your nights in Rome to the end of the trip? That would resovle your dilemna.

Multiple layers of folks in and out , we will have seen Florence and touring Rome must be at front end of trip.

Sounds like you'll have a car for the trip back to Rome, so maybe a night at one of the lakes (Trasimeno or Bolsena), Tarquinia for Etruscan sights, Tivoli for Hadrian's Villa and the Villa d'Estate, or even Pitigliano which is a stop on the new RS Best of Tuscany tour that sounds interesting. Without a car, your options will be somewhat limited.

In Porto Ercole, I enjoyed staying at this family-run, 2-star hotel: https://www.albergolaconchiglia.it/. Very friendly - for instance, the owner stood in the street to hold a parking spot while I pulled the car around.

Another town where I haven't stayed, but marked on my map for occasions like this, is Santa Severa.

It is not an overnight trip, but a pleasant way to spend a last day in Rome before departure, would be Ostia Antica. One could spend a relaxed day there and have time for a final special dinner in Rome the night before you, presumably, head home.

Thank you everyone!! We have new ideas now to talk about and book!