While I don't care for the Duomo inSiena, it has many other beautiful and interesting sights. Within the Duomo is a library of hand-painted (illuminated) books and paintings on the walls and ceiling. It is a beautiful room that I could stay in for a long time and just gaze at the walls.
There is also the Campo, which is the town square. It is surrounded by buildings and every year they host horse races in July and August with the 17 neighborhoods competing to be first place. The riders go three times around the Campo and ride without saddles. A horse can win even if the rider falls off. Some pictures of the Campo:
Siena is a hill town, so everything is up or down. Here are a few street views and then from up on a tower of the landscape below.