Also, try to get the 'F' seats on your flight out so you are on the right hand side of the plane. That is usually the one that you get the view of Venice from when you come into land is a great start to your break.
Definitely agree with going to Murano. Also, let yourself get lost - it is much more fun wandering around and you get away from the hoards. There are a number of supermarkets hidden away and these are great for getting soft drinks to refresh yourself in the heat rather than pay the expensive...