When in Rome

...what should you see?

I have two parishioners who are going to Rome to celebrate their twentieth wedding anniversary this November.

They're going on a tour but will some free time and have asked for suggestions as to what they should see.

I've already suggested the following:

You must visit the Basilica of San Clemente. There's also Santa Maria della Concezione de Cappucini, aka "the Bone Church." And don't forget Santa Maria in Trestevere.

You must have gelato. Every day.

What am I missing?