You can do a day trip to Lido di Jesolo if it's the summertime and it's just a boat ride from Venice. Or if you have a car, I suggest visiting the Euganei Hills which are 30 minutes outside Padua for some nice hiking and eating at farm restaurants.

Hard to choose just one! The distance between Venice and Milan is less than 200 miles and includes some lovely parts of northern Italy: Padua, Vicenza, Verona, Brescia, Bergamo, plus beautiful Lago di Garda. Any chance you can extend your trip, as all of these places are worth a day each at least!