Most visitors will want to take the vaporetto (water bus) down the Grand Canal to Piazza San Marco, see the piazza and the basilica, and walk up to the Rialto and back to the station. You will want to make this circuit, too, but do so in the early morning, before most tourists have finished their breakfast cappuccinos. Also, choose weekdays, instead of weekends, to visit the city.
Away from San Marco and the Rialto, the streets and quays of Venice’s beautiful medieval and Renaissance residential districts and important artistic monuments receive only a moderate amount of traffic. Even on weekends you probably won’t have to queue up to get into the Accademia museum.
Venice proper is quite compact, and you should be able to walk across it in a couple of hours. The water buses will save wear and tear on tired feet but won’t always save you much time.