What is there to due in this city?
Compared with Venice, Florence and Rome, it is a lesser city to visit, but there is enough there to justify a day or three on an Italian vacation itinerary:
Visit the Duomo, a beautiful cathedral.
See da Vinci's Last Supper.
World Class shopping among some of the most style conscious people in Europe.
Take a day trip to Lake Cuomo.
See an opera if you are into that. Or else just tour the building, one of the most opulent anywhere.
Milan has little to recommend it. It is a commercial city with almost no parks and little beauty. Spend as little time as you can manage there. It is an hub for trains to Lake Como, Verona, Venice and Torino--all of which have much more to offer, especially Varenna on Como and Venice and Verona. Vicenza is a worthwhile day and is on same line, as is Desenzano on Lake Garda