Tuscany - 4-6 days

Coming from San Marino, I would do a day trip on the way to Siena (which is perfect for centralized Tuscany day tripping) via Urbino, Gubbio and do an overnight in Cortona or Assisi depending on your interests. The next leg would be via Montepulciano, Pienza and either Montalcino or the Crete Senese Trail to Siena. For my wife and I, Siena is wonderful at night but crazy with tourists between 10am and 3pm. The "musts" for us are Florence (if you've never been there), Volterra, San Gimignano after 3pm, and Tuscany itself. It is our favorite place on Earth. The biggest "must" of all though is getting the RS guidebooks for these areas - read them before you go, take them with you at every sight and plan your off-sightseeing times as, ie, the "bra" bar in Volterra and the Brunello wine tasting in a 12th century fortress at sunset in Montalcino. Rick specializes in budget experiences to maximize your trip dollars too. I cannot emphasize this enough.
On your way to Rome, I would suggest going via the Val d'Orcia to Bagnoregio and Orvieto. Happy traveling.

I would pick a home base and then drive on day trips from there. We did something similar in May - we stayed just outside of Montalcino at Rick's recommended Agriturismo la Crociona (highly recommend it! We loved our time there!). They had private, covered parking spaces for each apartment. If you stay in a city like Siena or Montalcino, parking every night will be a huge headache (and will cost you). Staying at La Crocionoa, we drove all around Tuscany and did day trips to Montalcino, San Antimo monastery, Siena, etc. It was a great and central home base.

Plus, you're in Montalcino, which is home to Brunello di Montalcino, which means you could possibly be happy just sitting right there and drinking amazing bottles of Brunello all 4 to 6 days. :)

With a car I would avoid the cities and explore the countryside. You could always return to the cities by train. If you find the Tuscan village of your dreams, stay. If not, pursue the dream in another village...

I hate the logistics of checking in and out -- we have stayed in Tuscany to tour 3 times -- twice for a week and once for two weeks always with an apartment base. We really liked Montepulciano as a base because it is located in a beautiful area near many other nice towns and the town itself is full of good restaurants that you can walk to in the evening. It is a gorgeous little town although like all pretty hill towns has a tourist economy in the city center. I know the secret of getting a fresh loaf of bread in the morning in the historic center but it really is a secret; you have to work to find out.

If you are around Volterra you will be there for the medieval festival Volterra 1398 all week. The big day is sunday the 24th. I will be visiting this year on the 24th for it.

http://www.volterratur.it/en/volterra-a-d-1398/

You're on the right track with the advice above.