Rome Siena Florence

First of all, may I say how refreshing it is to see someone who is a first time visitor and NOT trying to see the whole country in one go! You will easily be able to not only see what you want to see, but do so at a leisurely pace, and just enjoy the ambiance that each city has to offer.

IMO Rome deserves at least 5-7 days; to explore it's many historic and cultural offerings. Florence deserves at least 3-4 days if it's own. But it is also an excellent base for day trips to places like Pisa, Lucca, and Fiesole. Siena can also be done as a day trip, or you could spend a couple of nights there.

But I think you also have the time to do one or two other places as well. Venice is the obvious add on, and 3 nights would be good there. Or, if you are up for renting a car ( and reading the rules for driving in Italy and avoiding the dreaded ZTLs), then a visit to come if the Tuscan hill towns with a stay at an agriturismo might make a nice change of pace after all the cities.

If you haven't done so already, get a good guidebook on Italy and watch some of Rick's videos on Italy to see what really piques your interest.

I actually spent five nights in Siena, and loved it. Did do a day trip by bus to Monteriggioni. There are lots of good possibilities for day trips by bus.

Florence would be a good choice for a base for 5-6 days. Easy day trips to Pisa and Lucca. Siena is a nice 3-4 day town and a hub for more day trips or even 1-2 days in Montepulciano. Rome for the rest. Get a few guidebooks to help you narrow down what to see and do there.