I'm sure you can fly nonstop to Milan or Rome or something. You could transfer and fly into Venice or nearby Treviso, but you're going to end up on a train anyway so it doesn't really matter where you fly in. If you fly into Rome, you can visit for a day, eat some great food (many dishes are regional and you won't find them in restaurants in other cities) before you hop a train to Florence, and then on to Venice. Or if you fly into Milan, I would suggest taking a train to Genoa and then along the Italian Riviera and Cinque Terre, on down to Pisa, Florence, and across to Venice.