Jay Y.
Yelp
If you want to see the local side of Venezia, then wake up early in the morning to see the local market by the Rialto Bridge. Go early enough to the marketplace on the right day, and you may be among the seagulls that are following the fishing boats as you ride down the Grand Canal!
Depending on the day of the week, Mercato di Rialto is either primarily seafood or produce. While I cannot vouch the seafood's price, the produce were actually fresh, cheap, and delicious!
However for outsiders like me, the biggest draw of the market may be the seafood, even though I could not buy them because I had no kitchen. Caught fresh from the lagoon, vouched by the eager seagulls that followed from the lagoon, the Venetian seafood is an exotic sight! While few common fishes like salmon are recognizable, most are simply bizzare. Simply being there to see them is a feast for your eyes, and certainly gives many photo opportunities!
Venezia isn't all about Piazza San Marco or canals, so even if you do not intend to buy anything, I highly recommend swinging by Mercato di Rialto to see the other side of the floating city. This market is one of the many reasons that make staying overnight in Venice worthwhile so you can see the Venice that daytrippers miss.