The Duomo in Florence is really a giant piazza (a square or plaza) around a complex of buildings - maps show it as the Piazza del Duomo, Cattedrale di Santa Maria del Fiore (cathedral) and it's museum (opera museum), Brunelleschi's Dome, Battistero di San Giovanni (baptistry), the Crypt of Santa Reparata, and Campanile di Giotto (bell tower).
These buildings are amazingly elaborate!
This is a gigantic mural made from different colors of stone
The floors are inlaid stone
The scale is hard to encompass
The inside of the dome:
A detail of the doors going out
More incredible stonework
See more pictures (of course) in the Italy album
Even though it's Monday, when almost all Florentine museums are closed, the Duomo is open. You can buy a tour for 15€ that includes a climb up the tower and also the dome, as well as entry to the Battistero and museum, or you can stand in a fast-moving line to see just the inside of the Cattedrale. Of course, anyone can just walk by the outside of the buildings :-)
We saw a few people turned away because their clothing was not appropriate - you need to have upper legs and shoulders covered. Many women opt for a shawl (often for shoulders, but at least one had her shawl wrapped around her waist to cover her to knees).
Since we have both knee and height issues, we opted for the short version.
We left our apartment at 10:30, walked around to the West side of the Cattedrale (signs are prominent and easy to understand), and got in line. The Cattedrale opened at 10:00; we were inside by 10:40. We strolled around, and were back outside by 11:00.
Plenty of hawkers will explain that you can wait in the line for an hour, or buy a ticket (from that fellow over there) for everything (including priority entry). Fortunately, we knew what we wanted to do (and not do) in advance. And we suspected that the line wait was exaggerated...