Start in Milan and spend a quick two days to see the Last Supper, the Duomo and then rent a car and visit Lake Como (Varenna is my favorite town on Como).

From there, head to the Dolomites. I go between towns, do some light hiking, take the cable cars and do a lot of lounging. Rent a convertible (or at least something with a sunroof) for sure, and drive the Strada delle Dolomite. 

And finish it up by staying at a nice hotel on the Grand Canal in Venice (The Danieli is amazing). I've done this a few times with extended family, just our family as well as together with my wife. Never gets old.