Brunelleschi's Duomo is a feat of architecture, sure, but it also makes an ideal observation deck. If you can make your way up the 463 steps (no elevator, and some passages that would make the claustrophobic run screaming), you'll be rewarded with pristine, 365-degree views of the city and all of Tuscany stretched beyond. For a view of the Duomo, climb Giotto's Bell Tower next door instead.