Best day trip from Venice

Padua (aka Padova) is an easy train ride, with the amazing Giotto frescoes in the Scrovegni Chapel, the museum attached to it, the Basilica of St. Anthony, and other medieval and Renaissance buildings. Alternatively, you might want to explore some of the islands in Venice's lagoon, like Murano, Burano, Torcello, and the Lido, all easily reached by vaporetto from the city.

Another possibility is a trip to Verona. Lots of history and things to do.

Padua, Vincenzo or Bassano del Grappa
Verona or Bologna

Just depends on what sort of day trip you want

You can't go wrong with Padua, Verona or Vicenza. Depends on your interests. Vicenza is probably the least mentioned of the three on this site, but it's a great one if you're into architecture. The Palladian buildings there are awesome. And there's the Teatro Olimpico there as well - https://en.wikipedia.org/wiki/Teatro_Olimpico - one of the coolest theaters I've been to.

Padua is also great - easy to wander, lively and the Scrovegni Chapel was gorgeous.

Bassano del Grappa is not far from Venice by train and you are transported into an Alpine village.This is our favorite day trip from Venice and we have taken quite a few.

Yes to all the places mentioned but how about this one: Trieste!?! It can be done but trust me, you'll want to return for a few days. Maybe even take a step into another country (you only need to take one step it's that close). Train service is direct, but ok, it's a couple of hours each way. So, you got scenery on the way there and a nap on the way back! Trieste is a great discovery!

Treviso
Padua
Vicenza
Bassano del Grappa
All these cities/villages can be easily reached by train from Venice

Really close, and pretty important, both for art and for history:

https://en.wikipedia.org/wiki/Villa_Pisani,_Stra

Some of my favorite nearby locations have been mentioned. I also went from Venice to also stay in Ferrara which is an hour by fast train or 90 minutes by a regionale. Ferrara has a castle in the center of town which I recommend, plus some nice museums. I especially enjoyed the National Archaeological Museum with the items from the ancient city of Spina.

We ate at Trattoria Da Noemi in Ferrara, and everything was excellent!

And in Venice, my favorite gelato is Suso - yum!

hey hey david
ilburchiello.it
boat ride up the brenta river to padova. check the tours, passing by the venetian villa along the way. train back to venice from padova. look at the times/days they do this
visitproseccoitaly.com
we did a tour with orellana, took a train to susegana and was met with driver/guide to travel prosecco road and the gorgeous sights along the way. driver was born and raised in this area. stopped at winery and tasted so many different local proseccos. stop at area with machine in vineyard that you can put money in and pick your bottle in vineyard. we also stopped for lunch at a local small restaurant, but things have changed for prices from half day to full day. our group of four friends split the cost which was few years ago. bought some local prosecco and brought back to apartment in venice to enjoy. didn't want anything i could buy in states.
take a train to treviso, home to tiramisu. walk around this small "little venice" stop at a tourist kiosk and see what they offer with small/short tours around the town. or do it yourself, have lunch enjoy people watching, have a spritz and tiramisu for dessert. train back to venice.
whatever you do enjoy yourself and have a great time
aloha