I received so many wonderful tips, thank you very much, from my original post that it raised more questions! We would so appreciate your thoughts!
Here is what we've planned and what is still TBD.
We have 17 total nights. Leaving in late September.
We will land in Rome where we plan to spend 4 nights. Then we will travel via train to Naples and take a car service to Sorrento for 4 nights. Then we are thinking Positano for 2 nights. OK, there are 10 nights, with 7 left to spend. We know we want to spend the last three nights in Venice (from where we fly back to the US). So we have 4 nights to play with. We are wondering how to best spend them. We loved Cinque Terra, but do we want to add another travel day and go there for a couple nights? Hmm. Do we extend our stay along the Amalfi Coast? Drop down into Sicily? What would you do?