Ditto on campo Santa Margherita.
On a trip last May, we arrived at our B&B in Dorsoduro later than expected due to our delayed flight. It was about 10:30 pm. and our quiet neighborhood was, well, deadly quiet. We were excited to be back in Venice,however, and secured a key and went out for a little wander. We were just about to call it a night and go back when we turned the corner into the campo.

Still half a dozen places open, people walking around, quite lively. We had something to eat, a glass of wine and enjoyed people watching.

On previous trips I recall getting a drink around the same time in Piazza San Marco, and actually going out to hear live music (a British band) at a blues bar.

But I like the quiet (except for echoing footsteps and water lapping), dark and mysterious Venetian nights the best!