This cathedral is an exciting place to visit. No matter how many times you see it, you always notice something different because it has so, so much beautiful and important stuff inside and outside as well.
The marble floors are a famous masterpiece; they cover a vast area but in the past five or so years some have been covered in order to protect them. They rotate which scenes are uncovered for viewing although there is often a time each year when they uncover them all. The scenes are poignant and relatable such as the one where King Herod has all male children murdered. They are also head-scratchingly hard to imagine how the work was actually done. The marble is inlaid in colors to form the amazing scenes. It is also interesting to reflect on the centuries of wear from people walking across them. (and that's just the floors)
Each chapel and side chapel is spectacular and contains spectacular works of art. Because it is so immense, even a stroll around the perimeter will take time. The frescoes, the vaults, the marble, the statues, gates, balustrades... goodness gracious!
Note: There is no fee to enter but there is a fee to visit the Piccolomini Library Pay it! When are you going to have a chance to see it again. The same for the Baptistry ad for the Museum of the Works of the Duomo. GO!