Hello caseyk.lake, and welcome to the boards!
You have a restricted schedule of 6 nights and the last one is in Milan so you have 5 nights to play with so if you want to try and see two cities/locations then 3/2 is the obvious way this would seem to work out. I personally love Venice and would advocate 3 mights there - especially because that includes your landing day/night so it shouldn't count as a full day.
As for a second location there is a list of obvious places accessible by fast train and then fast train onto Milan when it's time.
- Bologna is short on big sights but is the culinary capital of Italy so would certainly provide food and tourist options for a couple of days.
- Florence is the most obvious choice and has sights and food enough for a week so two or three days would just scratch the surface.
- Verona is somewhat out there suggestion, but is a pretty, genteel city with a Roman arena and theater (still in use) and is a very pedestrian friendly, arty, cafe culture kind of town.
There are smaller towns that might work - Lucca, Modena, Pisa, Siena - that are all with an hour by train of one of the bigger cities but unless you have a real affinity for a small town I think I would stick to a big city on a such a tight schedule.
There are fast trains for all the big northern cities. There are two fast train companies in Italy - the state run Trenitalia https://www.trenitalia.com/en.html and the private .italo https://www.italotreno.it/en
The earlier you buy you fast tickets the cheaper they will be so avoid buying "day of" if you can. Because they have assigned seats it is possible for these trains to sell out but that would probably only be on the busiest days of the year. Check the details of the different tickets - the cheaper the tickets the more restrictions about changing them later so you're trading price for flexibility.
If you come into Milan late and are heading to the airport the next day look into business hotels right near the Milano Centrale train station so you can walk over in the morning.
Hope that helps - have a great trip,
=Tod