8 days in North Italy

You can do it either way: it's super easy to get just about anywhere by train in Italy. There are some good day trips from Venice - I doubt you would run out after a week. Or find other places you wish to visit and train to them (or one of them) then train back to Venice for at least a night or two before you fly out.

Day trips: Burano (island off Venice - takes about 45-60 min by vaporetto boat each way to get there, colorful houses).
Verona
Padova
Bologna
Trieste
Ravenna

Further afield, you could spend a few days in Florence if you want (day trip to Siena). So maybe 3-4 nights in Florence and 3-4 nights in Venice. You could still day trip once or twice from Venice, too.

For trains, see www.trainline.eu . But there are lots of trains to all of these places.

A couple of suggestions:
If you go to Padova, and visit the Scrovegni Chapel, be sure to make a timed ticket reservation in advance.
If you are opera lovers, you could go spend 2-3 nights in Verona and attend the opera in the arena.
https://www.arena.it/arena/en

Normally I would say stay overnight in other locations for a couple of days and spend your last two days in Venice. But, if you’ve been on a cruise, how much luggage are you bringing? If it’s more than one suitcase that you are each able to lift on and off a train yourselves, I’d stay overnight at Venice and just go for some day trips to Verona, Vicenza and possibly Ravenna or Padova.

Although Ravenna is one of my favorite places, it's at least 3 hours from Venice by train each way. Ferrara takes one-half to one-quarter as long, and several people on the forum speak highly if it.

Venice is not a good base for day trips; unless you are staying near the train station (not the optimal place), it will take you a long time to get to/from the station, Venice is one of the most expensive places to stay, and most train trips are too long.

If you aren't spending time in Venice before the cruise, I'd suggest your last 3 nights in Venice, only 1 night if you'll have time pre-cruise.

Without going too far afield, you can base in Florence or Bologna (or split between the two). Florence has wonderful sights for those who enjoy Renaissance art and architecture, with good day trips by bus/train to Siena, Lucca, San Gimignano, Pisa. Bologna is a very pretty town with a some good sights and because it's a rail hub, many other towns are very easy to do as day trips - Padua, Modena, Ferrara, Ravenna. It's easy to find central lodging in either city that is walking distance to the train station (5-15 min) and both offer good restaurants for dinner after your day tripping.

If you have a lot of luggage, pack what you need for your days in Italy in small bags and leave the rest of your stuff at the Venetian hotel where you will spend your last night/s.

I agree with the suggestion of Bologna. Agreeable and interesting city, and a good base for all the mentioned smaller cities around.
I advise you to stay close to Venice on the one or two days before your return flight. Padua (Padova) would fit the bill: nice lively historical university city with a direct bus connection to Marco Polo airport.

Thank you for all of the great suggestions. Looks like we have decisions to make - will probably be Venice then base in Bologna.