Venice -> Lake Como -> Cinque Terre -> Florence. What to see in between?

You could stop for a night in Turin. It's a big city, but the core isn't that big. It feels big because it was once the capital of Italy before it moved to Rome. Not sure one night will really do it justice.

Or if you want something smaller, spend an extra night on the Italian Riviera, just south of Genoa. I love the Cinque Terre, but it's very touristy and busy. Consider a stop in say Camogli, a charming little Italian tourist town that doesn't get many American tourists but gets mobbed with Italians in summer. It will seem quite a contrast to Vernazza, for sure. Rapallo is just south of it and slightly bigger, nice too. With only one night there isn't much time to see much necessarily (there are some hiking opportunities but with only one night doubt you'd have time) but at least you'd get one more night on the ocean but in a different town. Your train will pass through anyway so it's just a stop-over. (Some trains won't stop in Camogli, I think more stop in Rapallo.)

You could stop in Genoa too - that's also probably going to be on your way and you may change trains there anyway. Genoa has some charm here and there and definitely has its champions, but it can feel like a big sprawling mess from many vantage points.

My personal bias would be to skip the night in Milan entirely and do two nights in either Turin or added to the Riviera instead of just one. Do a day stop in Milan instead and see the highlights you want to see e.g. Last Supper, unless there is really more to Milan you want to see. I am sure Milan has its champions too but I really wasn't crazy about it.

Turin, really, why? I’ve been there multiple times and don’t understand what the attraction is. Turin is a big, dirty, loud, industrial city with great food. If going make sure you take a fast train and not a regional train because a lot of pickpockets ride the slower trains between Turin and Milan. Furthermore, it’s out of the way and a bit of a hassle for one night.
Why not spend an extra night in Florence and take a direct bus to Siena for the day (1h 15m)? Make sure you visit Il Campo square and the duomo.

MaryPat, do try to understand that some people like places you didn't and need not explain why. We all have different tastes. I enjoyed Turin and most definitely think it's worth a visit. I have no idea if the OP will like it or whether it's worth the time. Just an idea.

Dear blk,

Answering to your original question, you could add a night to Venice and make a day trip by train to Padua. Or add a night to Florence and make a day trip by bus to Siena, to get a glimpse of the Tuscan countryside on the way.

If you are interested in World class museums, you should visit Milan and not Turin. There are more masterpieces in Milan's Brera Museum's first room than in all Turin's museums. And I mean real Masters like Mantegna, Caravaggio and Raffaello.

Nevertheless, the above post is ridiculously wrong, to the point I doubt it's based on a real experience.

Imagine a tourist walking Turin's 18 kms long baroque porticoed streets in the city center and thinking: "This is really an industrial city. Look a castle-shaped factory in the main square, they also built a Royal Palace-shaped warehouse next to it. That's not a real Baroque dome up there, it must be a giant trash bin". And no, regional trains in an area with almost zero tourists are not plagued by pickpockets; it doesn't get a genius to realise why.

Agree that either an extra night in Venice to visit Padua, or an extra night in Florence to visit Siena are your best bets. If you don't mind frequent hotel switches I'd overnight in those places rather than doing it as a day trip but that does mean booking and finding and moving to an extra hotel. Siena would be my first choice.

Although I enjoyed my few days in Turin I agree it is not in anyway comparable to Siena, Padua or about a hundred other places. Sure some people like different things, but based on your overall itinerary I'd guess you have more 'normal' tastes in terms of what you want to see in Italy so I'd skip Turin until you've been to Italy ten times and seen a lot of the rest.

Verona is a great place to go near where you will be! Spend a couple of nights, go to the Roman colosseum for an Opera or other musical production, wander the old city. It's a magical place.

With the transfer from Venice to Varenna taking 5ish hours, your arrival day there will be short on doing a lot of touristy stuff, so adding the extra night here would be my choice.

Is the one night in Milan to facilitate a visit to the Last Supper? If not, I would consider spending that night somewhere else. You could add a night to Florence and spend a day for a trip to Siena or Pisa and Lucca.

Nevertheless, the above post is ridiculously wrong, to the point I doubt it's based on a real experience.

My first time in Torino I noticed that I was being followed so my aunt and I stepped into a café and ordered delicious spinach sandwiches hoping the guy would move on. We took our time eating but when we left the café, that same guy was standing outside and continued to follow us. Fortunately, our hotel was nearby and we made it back quickly.
My second time in Torino I arrived on a regional train with luggage. I felt the power of the eyes upon me from a guy who looked suspicious as he stood near the door and stared. When the train stopped, he did not get off but walked down the aisle past me as I clutched my belongings.
The third time my sister and I rode a regional train into Torino and were prepared after my previous experiences and sure enough there were about three people standing near the door and when the train stopped no one got off but walked down the aisle instead.
The fourth time in Torino I went with my cousin’s wife who was on her way to work so she asked her friends to stay with me. That time was fine. These are my memories I have of Torino and oh, two of my cousins work at the Fiat plant in Torino.