Many great suggestions here. If you plan to spend at least three days, research the better restaurants and interesting sights away from Piazza San Marco and the Rialto, don't bother going to Venice. The city's charms are like the heart of an artichoke. You have to peel away the outer leaves to find them. If you just show up and try to wing it, you'll be wasting your time.
P.S. San Gimignano teally is mot worth a special effort unless you're a history buff. Tuscany, too, is overrun with tourists. Umbria is a far better choice.