You’ll definitely want to leave time for Rome’s best preserved ancient structure in Piazza della Rotonda: the mighty Pantheon, dating from the reign of the emperor Hadrian in the first century AD. Why best preserved? Because this former temple to the “pagan” gods has been an active Roman Catholic church since the 7th century. It’s majestic inside and out, but especially inside, with its soaring dome and oculus (central opening). It closes at 7:30 pm, by the way.
Read more in my post Rome in One Weekend? Sì, Signori!