People go to Venice to stay in Venice, to luxuriate in the city's distinctive spirit. The city feels like a cul-de-sac, a pathway to no other place. At no point will you feel this more acutely than when you arrive in Venice by train. As the train lumbers to its terminus at Venezia Santa Lucia, you can't help but feel that you've arrived at an isolated retreat.
It may seem odd to suggest day trips from a city that functions as a more or less enclosed space, but we've got some nonetheless: the island of Murano, known for its stained glass; Lido, the "other" Venice; and the distinctive border city of Trieste. Go to it!