Vatican City is an architectural wonder, a theocracy, a rich repository of art and literature, and the center of the Catholic church; there is a lot packed into the world's smallest nation-state. St. Peter's Basilica is the largest church in the world, and the interior is resplendent in marble and gold trim. The dome is impressive to see from inside the basilica, but I enjoyed the view even more from the tour I took to the top of the dome when I first visited over ten years ago. From the dome you can survey Bernini's masterful design of Saint Peter's Square, as well as the exquisite architecture of Rome spreading over its fabled hills. Nearby is the Vatican Museum with the Sistine Chapel and the wonderful Gallery of Maps, which was as magnificent as I'd heard. I highly recommend a visit to Vatican City on a Wednesday morning, when the pope gives an address to the masses assembled in the square, which are an impressive mixture of clergy, members of religious institutes, and visitors from all over the world. I've heard two different popes speak during my visits, and I'd submit this is about as quintessential a Vatican experience as you can find.

Logistics: Vatican City is an easy walk from downtown Rome, but it can also be reached by the Ottaviano metro station or by the Roma San Pietro railway station.