The Vatican is a surprisingly cool place to visit. Apologies to Michelangelo, but I think my favorite room was the Gallery of Modern Religious Art. Not a lot of pictures online (EDIT: Mama mia, oh yes there is, including this van Gogh. Who even knew van Gogh did religious art? Not me), and my time is short, but I can link to this image of the crazy Death Star-like giant metal ball that sits in the middle of the Vatican Museum. Definitely take the long route to the Sistine Chapel or you will miss a hella cool room.

Castel Sant'Angelo is pretty excellent too, and I get the impression not as many people go there as you'd expect. It's so close to the Vatican, and it's definitely worth seeing.

These aren't my pictures, by the way. I stole them all from the internets.

As for ruins, my brother's favorite ruin was the Domus Aurea (the Golden Palace of Nero), but I was partial to Hadrian's Villa near Tivoli. (This was about 20 miles north of Rome, so maybe he didn't get out there.) Hadrian was just about Rome's only good emperor, an artist, architect, and engineer who presided over 21 years of peace (the record for the Romans); you can read all about him here.

Onward to Florence and the Don's top Italian museum, La Specula. Arrivederci.