Northern Italy advice needed

There are no flights from Nice to Milan, so the train is your only option really (or bus, but it is less comfy).
Your time allocation is good, on the relaxed side. You could steal a night each from Varenna and Ortisei and spend 2 nights in Verona if interested.

As for the flight back: Milan has direct flights to the US unlike Venice, so it opens up a lot more flight options.

The official site of Trenitalia which is the state train company of Italy: https://www.trenitalia.com/en.html
They also have an app for use in country that also let's you track trains in realtime and validate tickets etc. Use the Italian city names for best results when searching on train sites: Roma for Rome, Venezia Firenze for Florence and get used to 24 hour times for train use.

.Italo is a private train company that runs only fast trains between big cities. https://www.italotreno.com/en

Local train tickets - called regionale - do not come with seat reservations, never sell out and always the same price so buy them when you need them. Tickets in the app need to be purchased a minimum of 5 minutes before the departure time and validated in the app a maximum of 1 minute before the departure time. Paper tickets are available from train machines and these should be validated in the stamping machines as you approach the tracks.

As always great information from the Man in Seat 61: https://www.seat61.com/Italy.htm

Venice can be a poor airport choice to fly out of for early flights because of the more difficult and uncertain nature of Venice logistics. If you are close to Milan then it is a good choice. The choices are usually good and while the MXP is an hour outside the city it is well served. Be aware there are multiple Milan airports - Linate is closer but less well served by mass transit.

Have a great trip,
=Tod

Currently getting from Nice to Milano involves taking a local train to Ventimiglia (runs every half hour) and connecting with an Italian IC train there. In the past there were direct trains, but these got axed during the pandemic.
Direct trains might return however. Both SNCF and Trenitalia have plans here. So you should look again once we know what next year's timetable is going to be.

We took a train in the opposite direction, Milan to Nice, and it took all day. We were very tired and cranky when we reached Nice, but that’s what going by train meant.

If more time was taken and an intermediate stay was included in the itinerary, that might make for a better overall experience.

Thanks for the helpful responses. After reading about the suggestion to visit Verona and doing some research on that, we've decided to extend our stay and spend 2 nights in Verona. With the new plan, which airport would be best for our return flight? Milan or Bologna?

Milan and Bologna are about equally far apart by train as cities but Bologna's airport is much closer to town and you can easily take a tram from the train station or a taxi from the city. The Milan airports are much further away - MXP is an additional hour on the train to get to the airport.

I suspect you will have more options and perhaps better pricing out of Milan but everything being equal I would choose Bologna.

Verona is a charming city, enjoy your trip,
=Tod

It's always my preference to fly direct on the over and back, so if there is a direct flight from Milan to Seattle that works for you, that would seem to make sense. If not, I would see what cities do offer a direct flight and get myself to that city from either Milan or Bologna by "internal" flight, train, bus, whatever. I mean, I wouldn't take the train back to Nice if the other comments are correct that it takes all day.

We took the train between Nice and Genoa. After several days in Genoa we then took a train up tp Mikan and then another train to Turin, which I highly recommend.

Thanks for all of the good information and suggestions! The info regarding train travel was helpful. After doing some more research, we have decided to fly out of Bologna per your suggestion, Tod. It sounds like an easier option, and the flights, at least at this time, are not much more expensive. As we get closer to traveling, we can always change to Milan if there's a drastic difference in price. There are no direct flights to Seattle from either city, so we'll go for more convenience in choosing our departure city.