Just walk: Rome’s historic center is easily explored on foot. I never take the subway, because otherwise I would miss all the beautiful things on the surface and, really, you only need less than an hour walk to get everywhere. I only recommend the subway to go to the Vatican Museums. It is very likely that you end queuing up to 4 hours under the sun because there is not a small shadow, so you’d better get up early and take the subway. The metro stop is Ottaviano. Do not stop at the Vatican Museum stop. Ottaviano leaves you closer to the queue. Look for the walls of the Vatican City on the corner of Via Germanico.
Enjoy the views: Everyone recommends the views from the top of the Spanish Steps, which are lovely, that is obvious. But I prefer going to the Quirinale. There you will find a square that is actually a bit soulless, but from there you will enjoy gorgeous views, especially at sunset. Romantic moment!!! Oh, and please visit Sant’Andrea al Quirinale, a church just a few steps away from there, designed by Bernini. Rumor has it that Bernini himself spent hours sitting inside it looking at his perfect work.
Go to the Vatican City State: I have to be honest here. The Vatican Museums are stressful. It’s like visiting the Louvre: there is so much to see that after two hours you just don’t know where you are, what to see and how to cope with all the information. But the Sixtine Chapel is a must! Like the Mona Lisa! And you have to climb St. Peter’s dome. You will get tired, but you will get the chance to admire the real interior of the dome. Just before you start climbing the dome, take a few moments to admire the astonishing beauty of the dome from within and look down: you will realize the proportions of the church. Yes, it’s that big. Before leaving the Vatican, send some postcards… After all, you are leaving a country!