Another vote for San Gimignano. Beautiful place.
Are you driving into Florence? Don't! They have something called the ZTL which basically bans all cars except residents, 24/7 all year round. Clicky Linky We drove in and 18 months later (yes 18 months) received a fine for 140 euro.. It's basically a tourist trap.
Venice, sit in St Mark's Square and enjoy a drink or a coffee. Very expensive but you are on holiday after all. Then go and get lost, just go for a walk down all the alleys you'll end up at a canal or the sea eventually. We spotted Gondoliers heading for lunch in a back street restaurant and followed them in, cheap, good food.