There are several areas where you can eat, drink, have fun and dance during the evening and the night in Milan:
The Navigli. A couple of channels used in the past by barges for commercial transport, now are the hearth of the Night Life, full of restaurants and cocktail bars.
At the beginning of the Navigli you will find the recently restored Darsena (dock) full of new bars and nightclubs, not so far from here the Columns of Saint Lawrence (Colonne di San Lorenzo), Roman ruins where people usually meet outside.
Not so far too from the Navigli, near Porta Genova Station, you will find Via Tortona. The area is full of fashion companies and restaurants of the quarter are of the same level.
Closer to the city center the Brera quarter mantains an international and bohemian vibe.
Near Garibaldi Station one the traditional night life districts, Corso Como, encounters the alternative and ethnic Isola area, formerly a working-class neighborhood, now very attractive and close to Porta Nuova skyscrapers.
At the end of Corso Sempione (one of the major avenues of Milan), there is a big concentration of restaurants, bistrot and bars, where in the evening Milan's people meets for the "aperitivo" (appetizer) which means drinking and pratically dining too.
Near Linate airport, the Idroscalo area (seaplane base) comes alive with discos in particular during the summer.