Advice on itinerary and Driving from La Spieza to Siena

Montepulciano could be a daytrip from Florence or Siena; Siena could be a daytrip from Florence as well but if you have time I think it's worth two nights. It has a different ambience than Florence, but that's harder to experience until the day trippers leave.

So, the reason we are doing 2 nights in Siena is mostly homebase for our tuscany drives and then experience the evenings in Siena. What about the driving?

I am not fond of two night stays, preferring three, but it's your trip. I would be inclined to suggest more nights in Siena, with a day trip to Firenze and Montepulciano and other Tuscan drives instead of so many short stays. Just rent a car for the last couple of days you are in Siena, making sure you have a hotel with parking. You can day trip to Firenze on the bus. Also add a night to Venezia as you will be jet lagged and it is a terrific venue for recovery. So your itinerary might be as follows. Italian names used as you need these for train reservations and to spot on signs while driving.

Venezia - 3 nights

Siena- 4 or 5 nights with day trip to Firenze on the bus (you could even go twice if moved to do so), and 2-3 days for driving around the countryside. Keep the car as far as La Spezia as the train tip between Siena and La Spezia is arduous

Cinque Terre - 2 nights. It is an easy train ride to Rome from La Spezia when you depart.

Roma - 4 or 5 nights

Can you elaborate more on what you mean by la spieza to siena is an arduous drive?

There are three trains between Siena and La Spezia. It's a lot of on-and-off, short connections, slow trains. What time of year are you going, by the way?

By the way, it's La Spezia.

This is your trip, therefore I don't want to suggest dramatic changes to your nights allocations.
One change that I would definitely suggest is to avoid changing hotels from Siena to Montepulciano. Siena and Montepulciano are very close to each other, therefore it is not worthwhile to change hotels, which is a time waster (check in, check out, repack, unpack again). Just choose a base in the Siena area and visit Montepulciano and any other town you wish from there.
One warning about staying in Siena with a car. The historical center is a large pedestrianized area closed to traffic. If you choose a hotel in Siena, you need to inquire about parking arrangements and if the hotel is in the traffic free area (basically the whole historical center).
The other change I would make is to do Tuscany immediately after Florence and before the Cinque Terre. In other words I would pick up the rental car in Florence, head out to the Siena area, and lastly go to the Cinque Terre by car, where I would return the car in La Spezia upon arrival. After the Cinque Terre, I would take the train to Rome from the Cinque Terre (one train change required in La Spezia, unless you stay in Monterosso, which may have direct intercity trains to Rome.

"The other change I would make is to do Tuscany immediately after Florence and before the Cinque Terre. In other words I would pick up the rental car in Florence, head out to the Siena area, and lastly go to the Cinque Terre by car, where I would return the car in La Spezia upon arrival. After the Cinque Terre, I would take the train to Rome from the Cinque Terre (one train change required in La Spezia, unless you stay in Monterosso, which may have direct intercity trains to Rome."

I 100% think we are going to take the advice of staying in Siena for 3 nights. The one question I have is why is the route that you mentioned better that original. They seem about the same, but I have never been there :)

We are going in April, first time in Italy and am very excited! Thank you all for your advice.

Sorry, I couldn't spell La Spezia*

You could do either way but since you are already near Siena when in Florence you might as well pick up the car there and do that first. Also that way you avoid the long train trip (3hrs) from Florence to the cinque Terre and go by car to La Spezia instead (1.5 hrs drive from Florence).

It's good we have the correct spelling now. It helps when you're googling around for info, to have the more or less correct spelling of these foreign place names.

Roberto thanks!

The small problem (which may be able to be changed since it is still early) is that we just booked the rooms per the original itin. I kciked myself after because I should have had the conversation earlier. With that in mind, the route we have set up Venice - Florence - Cinque Terra - Siena - Rome.....is that going to be more of a pain give the route, or should we be okay with the way it's setup...

Leave the itin as is. Not much of a difference. Just a long train trip from Florence to the Cinque Terre because of the two train changes (in Pisa and Spezia).
For the car return, instead of Chiusi, you might decide to go to Orvieto, in case you want to visit Orvieto as well. I usually return the car in Rome, but some people don't feel comfortable driving in big cities so it's up to you. But returning in Rome is easy if you return at the Tiburtina station. Just off the A24 freeway as you come from the Tangenziale Roma Est.

Thanks..another question...In regards to the driving, I understand there are toll roads and the last thing I want is to be hit be a fine from the rental company. How should I prepare...ie bringEuros's?