We visited Northern Italy from May 29 to June 19, 2008. We leased a new Renault and drove from Milan to Venice, down the east coast to Gargano, across to Rome, up to Florence, and finally back to Milan.
I think we chose the perfect time of year - the weather was perfect and the crowds reasonable. Having three weeks, we were able to see the famous sites, while still saving time to relax between them.
1) For a trip of three weeks or more, I definitely suggest leasing a car vs renting.
2) If you drive, get a Nuvi GPS with European map. This was a lifesaver!!!
3) Get the Rick Steves' Italy guide - and follow it.
4) Don't miss the Cinque Terre.
Please enjoy our photographic tour of Northern Italy.