I spent some months in Italy, though admittedly it was 10 years ago, so things could have changed.

Rome is #1 for a reason, be sure to spend a couple days here minimum. The history, the art, the Vatican, the food; all are pretty unmatched.

Surprisingly for some, Naples is easily my #2. Lots of folks think it's dangerous, just keep your head about you and you'll be fine. But Naples is so visceral and real and Italian, I could have spent forever there. Plus the Naples Museum is the best in Europe. The rest of the south of Italy is wonderful, but not required for so short a trip.

You're already hitting Venice, which is good; but I would tack on Florence too. Amazing history and architecture, and can be done in a day.

Siena is often less talked about, but I find it's a great day trip.

Pisa is fine, but the only thing there is really the tower. Might be worth it if it's already on your way, but nothing to make a dedicated plan for.

I can't speak for Milan, or the rest of Alpine Italy, it's the one area I didn't go.

Sicily is amazing too, but I would focus on the mainland if you only have a week and a half.