Some may disagree, but in order to leave time for finding surprises, getting lost, and just discovering the unexpected, for each full day you'll be in Venice (and Rome) choose one, maybe two maximum, must sees. Then if you see anything else it's a bonus, and it leaves time for wandering down side streets, getting lost, sitting at a sidewalk café, having some gelato or a glass of wine, etc. That will leave you plenty of reasons to go back when you have the chance, and you will want to go back.