The most famous square in Venice is that of St Mark, but if you're in the Dorsoduro sestiere, you should head for the Campo Santa Margherita—one of Venice's largest public squares.
The Campo was named for a church on the square, although that building is now used as a university lecture theater. It’s still worth a visit, though, and so is the “scuola” of Santa Maria dei Carmini. There is a regular outdoor market held in the square, as well as a supermarket that opens for visitors to wander through. But the Campo Santa Margherita is primarily known as a lovely place to relax during the day and as a hub of Venetian nightlife after the sun sets. There are several bars and cafes lining the streets.