day trip from Milan?

Lake Como.

Bergamo is easy and the historic area up on the hill is very atmospheric. There is a funicular or some such thing do you don't have to walk up.

Either the Certosa di Pavia or Lake Como or Lake Maggiore and the Borromean Islands.

Turin, If you buy advance tickets on the fast train they are inexpensive. There are many sights to see. The train station (PN) puts you right downtown and has an excellent TI right across the street.

A second vote for Bergamo: the medieval Citte Alta section above the more modern town.

A second vote for Lake Como. On a regional train from Milano Centrale it is about an hour to Varenna-Esino stop. Walk down to the shore and stroll around the lovely promenade in Varenna. Hire the Varenna Taxi boat to take you on a mid lake cruise, maybe including a tour of Villa Balbianello. Or take the ferry over to Bellagio for lunch. It is a quick 15 min ride. Head back when you feel like it, but be sure to purchase your return tickets at Milano Centrale. There is no ticket booth in Varenna, and the travel office that does sell them might be closed. Lake Como is drop dead gorgeous!

"A second vote for Bergamo: the medieval Citte Alta section above the more modern town."

Arriving at the station, purchase bus tickets, take #1 to Alta from the stop across the piazza. Going up the hill you will pass the Funicular [bottom of Alta], will get off at the top of Alta. Across the street there is a free funicular going further up for some nice views and a little park. Then you work your way down through Alta to the funicular. Then a walk or bus back to the station.

Thanks for help. I think I’ll take the train to Varenna then the boat on Lake Como to Como and back to Milan. Lunch in Bellagio and Dinner in Como sounds great.