Lake Como, Lake Maggiore, Torino, Piacenza, Bergamo are best visited from Milan.
Genoa is also most easily visited from Milan, although the distance by not so fast trains is such that maybe it should be an overnighter, depending on how much you want to see in the Genoa area. Torino is kind of big to be visited in a day, but it can be reached in a very short time from Milan thanks to the fast trains (lots of Torinesi commute to Milan that way every day since real estate in Milan is so much more expensive than Turin).
Parma, Modena, Ravenna are most easily visited from Bologna. Although Parma is sort of half way between Bologna and Milan, so it doesn't matter if you visit from either. Bologna is also very close to Florence (35 min) and Venice (1h40m), in case you are interested.
Lake Garda is equally distant from Milan and Bologna, however since the most beautiful part of the lake is the north lake, which has no railway and therefore you need a bus from Verona, maybe, if you want to visit Garda, you should consider a few days stay.
PS: I understand not liking to jump from place to place every couple of days, but if it were my trip, I would consider more than one base to visit all those locations, since some of them would call for multiple day trips. For example, I would consider staying a few days on the Ligurian coast, instead of doing a day trip to Genoa from Milan. I don't like Genoa that much, but there are some great locales nearby. Same is true for any lake, especially Garda which is too far for a day trip from anywhere except maybe Verona. Why not stay a few night at a lake of your choice? Turin too might need more than one day trip from Milan.
