Johnny gave great suggestions.

While you must see St Marks Square and Rialto Bridge, the magic starts when you just wander the back streets.

I have been to Venice 3 times, spent about 25 days total, and would go back in a heartbeat

Our favorite thing to do is literally get lost, grab some Quarto Formaggi pizza, and sit on some small out of the way bridge to eat it.

If you have time, go to the Island if Burano, and check out the tranquility there. While the go over to Torciello and check out the 14 century church. Pretty cool.