Another 'must see' in Italy?

Tuscany for sure. The hilltowns are wonderful. stay/see in Siena as a base and see the picturesques villages around by bus is easy and very affordable.

@JS thanks! how do I get to Siena? Fly into Florence and go from there? How many days do you recommend in Siena?

Other options in Tuscany from Florence are the San Casentino valley cities like Cortona or Arezzo. Both accessible from Florence by train.

Renee:
You mention Rome, Amafi coast and Sicily, so I don't think Tuscany is exactly "w/ current route." Tell us more about the France/Italy itinerary. How many days in each place and how many days to devote to one other place?

Try Paestum, especially if you're driving down the coast to Sicily. It's truly a remarkable place, and I'm amazed that so few tourists seem to go there, especially considering how close it is to Naples and the Amalfi Coast (about an hour south of Amalfi). Enjoy!

I'm doing 1-2 nights in Paris then 1-2 nights in Rome then 4 nights at Amalfi (or in the vicinity) and 3 nights in Sicily. Florence/Tuscany sounds great but now I'm actually considering SARDINIA. Does anyone has any experience going there? How would I get there from Sicily? How would I get from Sardinia back to Paris (I fly back to the States through Paris)? I'd arrange things so I can spend at least 3 nights in Sardinia.

You can easily bus into Siena from Tiburtina. It's about 3 hours. Otherwise there is a train with a connection.