Bridge of Sighs
You just have to. It's Venice. I will however say it's crucial to put your wallet in your front pocket and keep two hands on your bags. Watching clever pick-pockets try and find targets at this location is a bonus.
Caffè Quadri - Google Maps
Quaint little cafe that opens up to St. Mark's Square and is a great spot for taking a break and people-watching.
Murano Glass Company
Stopping by Murano it's amazing to see the glass blowing first hand and then tour the gallery of amazing art created here.
Church of Saint Martin Bishop
One of the most picturesque streets ending in 16th century Catholic church with a bell tower that has seen better days. Great photography spot.