Rome's subway is not terribly extensive, but it is easy to use and helpful for visiting certain places.

Fun with headless statues at the Baths of Diocletian, a museum that I would recommend skipping. The sculptures at the Palazzo Altemps and Palazzo Massimo still have their heads attached.
We trekked miles and miles to look through a keyhole at the Priory of the Knights of Malta, No. 3, ...

... and what we saw didn't even show up on camera. The dome of St. Peter's was framed perfectly by those bushes.