Many Italian cities have a beautiful cathedral, or duomo, often holding important works of art, magnificent frescoes, or intricate mosaics. Even if you're not interested in churches, visiting a cathedral can be very interesting. Most cathedrals are free but a few charge a small admission fee.
Take a look at these Top Cathedrals to Visit in Italy to find one that interests you.
Photo of Siena Duomo floor © by Martha Bakerjian, licensed to About.com