Use Rick Steves guides exclusively for my travels.
First, Rome now has monster crowds plus bumbling Italian security at the major sights. Waits can be hours.
I escaped on a Sunday PM to Villa Torlonia north of Termini but not that far from Metro stop. Calm, placid gardens, plus Villa and outlying buildings [including the Owl House] were a real treat. Gardens filled with suburban Romans, but the sights were delightfully uncrowded. No tour groups!
RIP OFF WARNING. The Ara Pacis museum has gone up to 10.5 E. Despite the tourbook's glowing recommendation, this was a 15 minute sight. The emperor's tomb which abuts the site on the east, is walled off with ugly metal fence and overgrown with weeds. Spend you time and money elsewhere!