You could tick off the major touristic sites in Venice in a couple of days. The Piazza, Ducal Palace, the Basilica, a Scuole or two and maybe a museum, if you are organised. However, you won't really see Venice in that short a time.
It's the second tier, the odd and quirky things that grab us. The photography museum, Tre Ochi on Giudecca, Biennale things if they are happening (the Biennales, whether Art or Architecture, run from late may to early November), the ice house in the Hotel des Doges in Cannaregio, those sort of things, and they have no equal anywhere.
You need time to wander around, to get lost and to find yourself, time to enjoy a spritz in a campo somewhere.