Take the train! It's faster, It isn't Amtrak! Milan to Venice 2hours 35 minutes. No need to stop for fuel, rest rooms, food etc. More relaxing. And you get to see the scenery - nobody drives Autostrada etc (European Freeways) for the view!

Probably cheaper - no fuel, no tolls, no car rental, no vignettes (Hungary, Austria and Switzerland). Train Milan to Venice around 24 Euros.

In the places you are going take tours, rent a car for a day.