Well amazon05, certainly you can expect the main tourist center of Venice (Rialto Bridge to San Marco and directly up to the train station) to be very crowded from about 10 to 4, especially on days when multiple cruise ships are in port. However, many other parts of Venice, and even the tourist center in the early morning and evening, can be very uncrowded and lovely. I myself would not want to be near Venice and not visit, or to stay in Mestre or elsewhere, and just visit Venice for the day.
Beyond Venice, Padua is about 30 minutes west by train, Vicenza about 60 minutes, and Verona about 90 minutes. Each is a lovely and historic town, with many sites and experiences, worthy of a day or a few days. There are also some very interesting small towns not very far from Vicenza to the northeast, reachable by train, which have been on my radar for some time, although I have not yet visited.
Of course there are also the lagoon islands of Murano (glass), Burano (lace, seafood and pastel colored houses) and Torcello (very rural birthplace of Venice) make up a great day trip from the city of Venice, although especially Murano are likely to be very crowded at that time as well.