IN my opinion you can spend one day in Milan and then move away.
I would prefer to rent a car and drive to Como and make the coastline to Bellagio (the real one!!), then take the ferry and cross the lake to Menaggio (nice gardens, nice walkabout) and drive back the other shore.
You can spend one day in Venice, and I suggest you to go there with the train.
If you want to visit a less known and less crowded city I would go to Verone, very nice city centre where you can walk around.
If you like skiing you can take a day trip to Madesimo or St Moritz (switzerland).If you can spend two days out of Milan go and see Dolomites in Trentino, some 3 and 1/2 hour drive, beautiful mountain and sky resort.
If need of any other help you may PM

Matteo