I will have an extra day in Milan. Which will be the best day trip from Milan?
1. Bellagio/Lake como
2. Lugano - Swiss
3. Bologna
4. Lake Maggiore?

What time of year? I assume you'll go by public trans...

If budget matters, the fare to/from Bologna is much higher than to the other destinations.

Another low budget option is Bergamo.

Parma is slightly closer to Milan than Bologna but it's still a direct train link. The center of Parma is compact and nice for strolling, and like in Bologna you can still get a great meal.

We are traveling in May end. No restrictions in budget/ transportation mode.

Yes,Bergamo is definitely a good option. The historic centre (Bergamo alta) is really nice. Have you considered Verona ? (a 2 hours train-ride from Milan)or Turin( 2 hours by train) ? Which-unlike Bergamo and Verona which are nice little cities, is another big city with a very nice historic center

Another option would be Como, just half a hour from Milan by train, or the very beautiful small city of Mantua.
Brescia, which is also very near to Milan ( 1 hour away by train), is also definitely worth seeing.
Still, if you have not seen Verona yet ,that would be the one I mostly recommend

Another vote for Bergamo. I loved it. Easy day trip from Milano. I also did a day trip to Parma but I found it very 'meh'. Bergamo was my surprise best day of the whole trip.

Mantova is another great town but not sure if the connections to there are more difficult. (I did that one from Verona).

http://www.pbase.com/annforcier/bergamo_mantova

I'm planning a trip in October and looking for a daytrip or two. Bergamo was also recommended to me by goldenautumn. It looks very interesting and now I'm putting it on my to do list.

Thank you! I will be travelling to Verona from Venice. From Milan, i guess the most recommended is Bergamo!!!