I live 100K north of Venice (Budoia, PN) and it's certainly possible
for a day trip to Florence and it's should be about 2.5 hrs from Venice.
Recommend to leave early and enjoy it!
BTW, Verona is another option (Riet Bots mentioned above) and it's another lovely town in Veneto region.
I personally enjoy smaller towns like Asolo, Bassano del Grappa, Vicenza and Padova and all are within a day trip options.
Buon viaggio e divertiti!!
pk from Italia about 5 years ago
If it is indeed 2.5 hours each way then as long as you take an eeearly train I would say go for it. Because I am assuming you have never seen Venice and you defiitely want to see Venice in your lifetime. NOTE that if it is high summer season then the main Venice streets are crowded terrible rivers of people and priority one should be to get into a small quiet neighborhood where you can sit and look at a canal and say ... I am in Venice. about 5 years ago