One day in Venice?
If you aren't using Air Bnb you should be using Hostel World, both are amazing for finding places to stay that are affordable while traveling.
We stayed here, http://www.hostels.com/hostels/venice/plus-camping-jolly/15562?source=AdwordsGlobalDSADeskHCEN&network=g&creative=239471698535&adposition=1t4&uniqueclickID=18310768502310598477&sub_keyword=&sub_ad=b&sub_publisher=ADW&gclid=EAIaIQobChMI4KjK6_Wa3AIVGLvsCh12YgaYEAAYBCAAEgKkmPD_BwE
Some things to do in Venice, take a bus into town and WALK EVERYWHERE! Sooo much to see, make sure you get off the beaten path and go early, it gets busy!
Must see and do: Basilica di San Marco, go on a gondola ride. Yes they are overpriced but they are worth it! Eats some gelato. Spend most of your day sitting in Piazzo San Marco, drinking wine and eating pasta.