With such a long laundry list of historical treasures, epicurean pleasures, and equally tempting outdoor resorts, the hardest part of planning a trip to northern Italy will be deciding where to spend your time. Fortunately, much of what you will want to see and do can be dictated by the seasons, and anyway, with such short distances between the cities, a change of heart might be only a 2-hour train ride away.
Getting around northern Italy is a breeze (minus the city and highway traffic and the occasional jam-packed train), and getting to and from the region couldn't be easier with a nice selection of local airports and a very busy international airport in Milan.
In fact, it is so easy to travel to northern Italy that the greatest hindrance might be that the region is so very busy. You'll have to plan far ahead to get a nice hotel room in Milan -- especially during the spring and fall fashion weeks and the design trade show in April. Self-catered chalets in the Alps and the Dolomites fill up many months prior to the weeks around New Year's Eve, and the choicest spots on the Riviera and Cinque Terre are snapped up before it starts getting hot. As for Venice, it's a general mob scene in any month not containing an R and during any of its spectacular festivals. The secret is to plan your trip months in advance, or at least secure accommodations and tickets to museums and concerts in advance.
Other than that, if you do your homework on special events, pick the right place for the right season, and pack for a variety of climates, preparing for a trip to northern Italy should be pleasant and uncomplicated.