Where to visit before sailing from the new ‘Venice’ cruise port

Take a look at Bologna?

But I'd also plan a night or two in Ravenna itself; go see the 8 UNESCO mosaic sites.

https://www.turismo.ra.it/en/follow-your-way/unesco-monuments-ravenna/

I would suggest 2 nights in Ravenna. You need a full day to see the mosaics—-photos online do not do justice to their stunning beauty. Palazzo Bezzi is a lovely but moderately-priced hotel within walking distance of the train station, so easy on arrival.

https://palazzobezzi.it/?lang=en

For your other 2 nights, in addition to Bologna, you could consider Ferrara, another UNESCO world heritage site:

https://www.ferrarainfo.com/en/news/10-reasons-to-visit-ferrara

Or spend 2 nights in one of the towns close to Venice, such as Padua, Vicenza, or Verona.

There’s a train that departs the Venezia S. Lucia station that requires a transfer in Ferrara (1h 15m) to get to Ravenna. I did not see direct trains between Venice and Ravenna nor do I remember there ever being any.
I would sleep in Ferrara for two nights before continuing on to Ravenna (1h 15m) where I would spend the last two nights.

Also, if not wanting to pack up and change hotels, Ferrara could easily be done as a day trip from Ravenna. No right or wrong to it but having a base can provide some flexibility to day trip according to weather; nice if intending to do a lot of walking outside.

My first thought was Ravenna itself because those mosaics are stunning, but Ravenna doesn't need four nights. Padua has many interesting sights and is conveniently located for day-trips to Vicenza and Ferrara if you want a change of scene.

The whole Veneto region is very interesting including Padua, Vicenza, Bassano del Grappa and more. We dedicated one trip just to the Veneto. None are far from Venice and would be a nice addition to your trip.

The train from Venezia S. Lucia stops in Padova (Padua) on the way to Ferrara (30-minutes). According to the internet you can leave your luggage at platform one where it says “deposito bagagli”.
In Padua I would buy tickets before leaving home to tour the Scrovegni Chapel and enjoy a stroll through the old town. Ferrara is another 30-minutes.

We spent a week in Bologna last month. Plenty to keep us busy, with sights and food opportunities.

If you wanted more unique experiences, Slovenia is just to the east. Rent a car, or find other ways to get to Piran and/or other towns. Or is your cruise going there?