A great day trip from Venice is Verona. It's about 1:15 hours from Venice and is the home of Romeo and Julliet. The city is very easy to walk and has so much history. The Collosseum there is fully functional and home to the Verona Opera Company.
Another great day trip from Venice is Cittadella. It's about two hours by train and is a wallaed Medival city. You can actually walk the ramparts all the way around the city! Nice museum and cafe's.