There is so much to see and do in Venice. For serious culture vultures with only a day St Mark's Basilica, the Palazzo Ducale, Grand Canal, and Peggy Gugenheim Gallery are must-see.
However, to do Venice justice you really need three to four days minimum to get a feel for the city and Venetian lifestyle which is as much a part of visiting the city as seeing the historic sites.
Exploring is the name of the game as you will get frequently lost amongst the myriad streets and canals which scatter the city.
Quick Tips:
If you fancy a day out off Venice, Verona is a wonderful day trip away and it's wide open spaces can be a welcome relief from the sometimes oppressiveness of Venice.
Best Way To Get Around:
Getting around Venice is easy once you get the hang of the Vaporetti, bus-boats which ferry you around the archipelago. On arrival, purchase a ticket for the vaporetti - a three-day day ticket costs about 18 euros for unlimited travel . . . if not, it's 3.10 euros for each trip which can work out quite costly if you average three or four journeys a day.
The vaporetti are also a good way to get a tour of the islands if you're on a budget and can't afford the over-nflated gondola trips, which might set you back about 70 euros for one hour!!
Otherwise, you should be prepared to walk - it's very easy to get lost and it takes a couple of days to get your bearings. During your wanderings, you'll notice some yellow signs which direct you along the main pedestrian routes. Try to keep within touching distance of these to begin with and you won't go far wrong.