Yeah, even if you don't go to the museums, St. Peter's square itself and the basilica are awe inspiring, plus you get nearby Castel St. Angelo. Totally not to miss, as "well, d'uh, I know" as it sounds, the Coliseum and the nearby Imperial Fora (plus the Capitoline Hill just behind), the Trevi fountain which is really beautiful as it looks, just overcrowded any time of the year, every day, every moment, and one of the most beautiful squares of Europa, Piazza Navona, followed closely behind by the spanish square (Piazza di Spagna), which has a beautiful staircase topped by a nice church.
There's a lot more to see but a couple of days are enough to see all these "greatest hits" which are definitively not to be missed when in Rome!