St. Peter's Basilica (Basilica di San Pietro)

The epicenter of Roman Catholicism, St. Peter's Basilica is centered in Vatican City and open daily for tours. But it's closed on Wednesday mornings for pope appearances. Many visitors enjoy trekking to the top of the dome. You can climb the 323 steps to the summit; for a slightly higher fee, you can take an elevator. Regardless, you'll take in spectacular Roman panorama.

Keep in mind that this is an active church with daily mass services. Likewise, a stringent dress code is enforced: No short skirts, hats or bare shoulders. And because St. Peter's Basilica is one of the area's major attractions, there is almost always a long queue -- though it tends to go fast. Recent travelers recommend you spring for a tour guide; the depth of insight they bring to the basilica really makes the experience.