Day Trips by Train Frim Venice

Padua (aka Padova on trenitalia.com) because of Giotto.

Vicenza for Palladio, the 16th-century architect without whom we wouldn't have the classical architecture of the White House type.

I know you said by train, but check this out.

https://www.venetoinside.com/tours-activities-in-veneto/tour/burchiello-cruise-among-brenta-riviera-villas/

We haven't been there (yet!), but many people seem to have enjoyed Bassano del Grappa. I'm also interested in visiting Treviso. We have been to, and loved, Ferrara - just the ambience, the castle, the walls, all the bicyclists.

I was going to suggest the Brenta Canal ride also. It makes for a long day if you want to get back to Venice the same night. Otherwise overnight it in Padua for the Giottos.
Padua also has the medicinal gardens.
Vicenza is a little further and has more Palladian homes.
Emillio Romana province, and the train line to Milan has so many interesting areas. To explore. You can see the Ferrari car factory or Lambourgini...but I would rather drive one than look at it in pieces.

I’ve visited Padova, Vicenza and Treviso and would recommend any of them for a day trip. And a little further afield, there’s Trieste.