Best way to get to Cinque Terre

I would park the rental car in La Spieza ; huge underground lot at the train station
Then take a regional train from there ; which is very close and those local trains need not be reserved in advance,

You might consider dropping the car in Pisa and continuing by train. A rental car in the Cinque Terre is about as useful as a boat anchor. Of the five towns, only Monterosso has decent parking.

Yes, you can reserve accommodations, and even for travel in September it would be a good idea to do that soon!. Monterosso would be the best option as it has the largest number of lodgings. Do you have a copy of the RS Italy guidebook? If you need suggestions for hotels, post another note.

For your ongoing trip to Rome, take the Regional train to La Spezia Centrale (about 15 minutes) and then one of the direct trains to Roma Termini (about 4 hours).

You can easily stay in Rome prior to your flight home, as you can use the Leonardo Express to the airport (about 32 minutes). With a group of four, a taxi will probably be about the same cost. Ask your hotel to prearrange that the day prior to travel.

As you'll be using a rental car, I assume you're aware of the need for each driver to have the compulsory IDP, which is used in conjunction with your home DL. You'll also have to be vigilant to avoid ZTL (limited traffic) zones as those will result in hefty fines.

If you are going to keep the car, I'd look at something near the vicinity of Tarquinia for your last night.

Thank you for your suggestions. The regional train sounds perfect to Cinque Terre. I will check out the IDP before we rent the car at the airport after a week in Venice before we drive to the Florence area. Any suggestions for a good night’s stay location between Venice and Florence? We will use the car to drive to Rome after Cinque Terre visit and wish to find lodging near Rome along the coast for a night or 2.

We drove to Monterosso and parked in the public lot there. We stayed at Hotel Porto Roca

Do you plan to visit Rome, or are you just looking for somewhere close to the airport (but with some attractions) for your last day?

We do not plan to visit Rome at this point, but rather be close enough to return car and catch an outbound flight at 3 p.m.
Thus, either an accommodation by the coast or an airport hotel.

Thank you for the suggestion of Tarquinia. Will check it out!

You could consider stopping in Padova between Venice and Florence, however the distance is not that great so it might be better to go directly to Florence.

One caveat though - DO NOT drive in Florence as the city is just about saturated with automated ZTL cameras. Each pass through one will result in a ~€100 fine, which you won't know about until several months after you return home. You may to have a look at this website - http://www.slowtrav.com/italy/driving/traffic_cameras_speeding.htm . Many towns in Italy have ZTL areas, so this is not only a concern in Florence. You'll also have to be careful to avoid speed cameras, driving in bus lanes, etc.

If you want to keep the car until Rome, the easiest solution would be to drop it at the airport and stay in the Rome Airport Hilton. It's a bit pricey, but you'll be able to walk directly from the hotel into the airport.

Thank you, Ken, for your caution on driving. It’s been 40+ years since I drove through Europe, including Italy! Are there auto parking areas on the perimeter of Florence from which we could use public transit?
Have you or other readers used train from Venice to Florence and up to Cinque Terre and south to Rome? Eurail or Italian trains? We’re planning on water taxis in Venice!

Curious if you have a good reason for having a rental car for your trip.
From the places you have mentioned going a car will only be trouble for you.

There are places where a car is better: Tuscan countryside south of Florence, Dolomites, anywhere south of the Amalfi Coast, I am sure other spots as well.
Florence, Pisa, Cinque Terre, Venice, Rome though are all much better to use the train (high speed trains in many cases will be faster than one could drive anyway)

I will check out the IDP before we rent the car at the airport after a week in Venice before we drive to the Florence area.

The IDP is something you need to obtain BEFORE you leave on your trip. Perhaps I am reading your comment incorrectly, but I have the sense you plan to get the IDP once you are in Italy. If I misread your comment, please excuse my being 'dense'.

Stayed at this hotel when we had an early AM flight out of FCO..... 3-5 minute walk to the sandy beach. Had no problem walking to restaurants

http://www.hotelrivierafiumicino.com/en/home-page.aspx

Yes, I've used train travel for all the routes you've mentioned. The Italian rail system is excellent, especially the high speed trains that travel at up to 300 km/h (much faster than any car and a better use of limited holiday time).

"Eurail" is only a marketing agency. Better to deal with the firms that actually operate the trains such as Trenitalia or Italo Treno.

I provided a suggestion in my first reply on travel from the Cinque Terre to Rome. The other are easily travelled also but train changes will be necessary on some. There are also some potentially expensive caveats to be aware of when using public transit in Italy. If you need more information on that, post another note.

Water Taxis are expensive but if your budget allows that's certainly a nice way to travel. You can also use the *Vaporetti" (water buses) which are less expensive.

Regarding the IDP (international driving permit) that needs to be purchased before you leave on your trip. It can be obtained at an AAA office or online.

Trains, will work for Venice, Florence and Cinque Terre. If you're staying at an agriturismo by Pisa, you will probably need a car.

Edit. You could drop the car in either Pisa or Cinque Terre. The other option is drive to La Spezia park at the station then drive to the airport to return your car.

Thank you, Joe, for the link. I have planned to get the IDP here in the States, likely AAA.