You really need to spend a week here. We did two, with side trips to Sienna, San Gimingano, Pisa and Luca.(first 3 WHS's) The duomo needs a whole day to be awed, one at the Uffici, one in Barboli Gardens, half a day in Academia to look at David and other Michaelangelos, one day walking around the banks of Orno. If there is one WHS you must see in the Western World..... I would stay at one of the villas across the river. Much more space for the money, and the walk is really nice. The hotels are expensive and bad.