One day trips from Milan
with or without a car

One day trips from Milan without a car:

You don't need a car to escape from the business, the traffic, the congestion, the fog in wintertime and the afa, the humid heat, in summer, of the city of Milan to a wonderful world of lakes, mountains, castles and good food: just take the train and, sometimes, the boat as recommended in the following itineraries.

The first set of itineraries cover downtown Milan,
and the immediate neighboring locations:
Downtown Milan on foot
Milan - Chiaravalle Abbey
Milan - one day with art and shopping

A visit to the old medieval town of Bergamo is worth spending one day:
Bergamo Alta

Como Lake is well within reach of a day trip from Milan:
Isola Comacina on Lake Como
Lecco and Bellagio by boat
Como and Bellagio by boat

A trip with the high speed train: experience the travel at 185 mph!

Bologna 

Garda Lake, divided between Lombardy, Veneto and Trentino,
is another destination for a one day trip by boat:
Garda Lake by boat

Pavia, the old capital of the Longobard Kingdom during the
Middle Ages, is well within easy reach from Milan:

Pavia, the city
Pavia, the Certosa

Portofino, the "Pearl of the Mediterranean".

Portofino in winter

One day trips from Milan with a car:

In less than a couple of hours drive from the city of Milan you can reach by car a wonderful world of lakes, mountains, castles and good food, via the "autostrada" (highway) the Valle d'Aosta waits for you with the following itineraries, that can be done in one day from Milan.

Official Tourist Boards - APT della Lombardia - Useful links | Travel Italy