Once upon a time, Milan was a city of water—way back in the twelfth century, a network of canals (maybe designed by Leonardo Da Vinci?) were built all over the city in order to connect the central Duomo to nearby lakes. Most of the canals have since been filled to make roads, but two, Naviglio Grande and Naviglio Pavese, still exist.
This is my favorite area of Milan—a place where you can imagine an Old Milan that looks a little bit more like Venice. The Naviglio Grande, in particular, is bustling with shops and restaurants during the day and bars (many serving the famous aperitivo) at night. Two of my favorite Milan spots, Mag Café and the Maison Borella, are located along the Naviglio Grande.
More information about the Navigli can be found here.