I have to agree with
@Idaeo on his post on Venice. I've been twice both times for just 24 hours. I'm can honestly say it's the only city I can say I've ever truly been lost and didn't know which direction was North from South.
Saint Marks Square is a must see. It is way too crowded though. Your best bet is to be there early before everyone else.
Make sure you ride the taxi that runs through the Grand Canal. It's a good way to see a lot of Venice.
Gondola rides are kind of expensive, but it's enjoyable. Your guide will point things out to you that normally you'd never see or know anything about. It's actually very calm and soothing when your gliding a cross the water away from the main tourist areas.
I highly recommend getting away from the main area and get lost. My favorite part was just wandering in the areas that most people never see.
If you have time and money (not sure how much it costs. It was part of a tour I was on) go watch glass blowing. It's pretty incredible what they are able to make.
Pics from my last time there.
Oh and if you see cruise ships coming in to dock get far away. Thousands and thousands of people are about to take over everything.