Where to visit from Milan

So, in a few weeks I'm going on a vacation. We're 3 people, it's gonna be a 7 day travel, and we booked an appartment in Milan for the whole trip so we wouldn't be carrying the bags all the time. The day we arrive we are going to explore Milan. The last day is to pack bags and go back to the airport. We are left with 5 days to visit other places.

We were looking into visit Florence, Verona, Lake Como, Lake Garda, Lugano and maybe do the Bernina train. This are all options to do on this vacation and we are trying to choose what to do, in each day.

For example, for me, Lake Garda, Florence and Lugano are priorities but I'm not the only on vacation. But we are flexible and have about the same tastes.

For people from there and who have traveled there, how would you advice organizing this, where to go, what advices do you have? Should we book train tickets right now before going or it's fine booking them on the spot? What about the boats if we go the the lakes?

Thank you in advance.