Itinerary feedback

I am tying to work out an itinerary and would appreciate any feedback. I will be traveling with my two teenagers.
We will be flying into Milan, staying one night and then taking the train to Florence and will be staying there for 3 nights. I cannot change this leg of the trip. We have 9 more nights and fly out of Milan. We want to make it to Venice and am trying to figure out if it is possible to also get the Aosta valley and a seaside town in on the Mediterranean? Is this too ambitious? I do not want to be traveling constantly. I am also open to renting a car. Alternately, I would visit Venice and the Dolomite's. What do you advise? Thank you!