This has probably already been said, but Venice is truly magical. People will say it is overrun with tourism, and that it's lost its charm. Don't listen to them. Of course tourists arrive en masse, and of course Piazza San Marco is flooded with people on peak days, but don't let that stop you from experiencing one of the most fascinating cities in human history, from a rich historical perspective and from an architectural and engineering perspective. The entire city is a work of art, and if you are a church history buff you can't miss St. Marks Basillica. The Doge's Palace though is the top sight for me.
One of the great things about Venice is that there are hardly any new buildings on the island-- just re-purposed marvels of Venetian glory days. The whole city feels "frozen in time" which is incredible and makes you feel like you have transported 500 years in the past. Rick's Pocket Venice has incredible tours of these facilities, as well as walking tours that were altogether engaging, educational, enlightening, and entertaining. My wife and I at times could hardly contain our laughter during some of his tours-- Rick's "dad" jokes are in rare form in his Venice book.
We went to Venice for 3 nights as a leg of our Italy trip this past May, and it was our favorite place. It was crowded at times, but the island is bigger than people think and has plenty of off-the-beaten-path hideaways just waiting to be discovered.
Just about everything in Venice is world-class, aside from maybe their food. I enjoyed it, being a lover of all things fish and sea (speaking of which, the fish market on market days is quite enthralling), but my wife was a little more limited in her food choices. But like I said, world-class top to bottom. History, culture, art, architecture, its all there. And it's of course incredibly pedestrian-friendly with no cars around. Strolling at night with the day-trip cruise tourists all gone is where Venice steals your heart.
The canals and boats and gondolas that come to mind when you think "Venice" speak for themselves, but the city really has so much more to offer and I wish we had spent a week there-- we didn't have time to visit the surrounding islands, which look exceptionally charming. Get an airbnb if you go there (in my opinion best value); there are some sweeeet digs in that city. Sleep away from the main drag; there's no point in being centrally located when the city is so easily navigable. I would recommend the Accademia region. Sorry for the long post; I am really a fan of the place!