I am going to be a bit contrary here.....
Others say to avoid the tourist spots, but there is a reason they are popular. Just go out at sun up and it is incredibly quiet. Getting pictures of shop keepers and delivery folks is great that time of day too. While the walkways are tight and a wide angle is useful, the details in the buildings is really incredible so a 120-150+tubes on your 6x6 will be great also. If there was any city I could photograph for 10 days, Venice is definitely it. Have a great time