I really enjoyed the different perspective I got by visiting the Duomo and Baptistry at night. The crowds were still there, but it was all dark and kind of cool to see all the colours of the buildings in the dusky light!!!
There are usually a lot of people just strolling around the duomo at night. So, u can either do the same or remain in your hotel ;-) The Piazza's like Repulica and Signoria and good for sitting and seeing the young and old going here n there.
Dress Code: anything warm and comfortable.