Suggestions for "UNDER THE RADAR" Rome for my fourth trip
 

Will be stopping in Rome early August for a couple of days before heading to Castelfranco in the Campania region for 11 days for an Italian "immersion" trip.

Have done all the big must-sees in Rome (like Coliseum, Trevi Fountain, Spanish Steps, Vatican, Appian Way, Catacombs, + more ) on my previous 3 trips and looking for some under-the-radar things to do in Rome this time.

What "Under the Radar" suggestions do you have?
We will be staying near the TERMINII rail station.

Unless you list everything you've seen in Rome, it's hard to know what would be "under the radar" for you. For instance, you don't mention Galleria Borghese or the Castello, any day trips, etc.

A couple of less mentioned attractions:

Domus Aurea
Ara Pacis and the neighboring Mausoleum of Augustus
Villa Medici
Palazzo Massimo alle Terme (part of the National Museum)

Sometimes wandering or a wrong turn takes you somewhere unexpected. On our last trip, we made a wrong turn getting off a tram at the Buenos Aires stop and stumbled into what we later learned was Quartiere Coppede. Beautiful Liberty architecture. Do a Google Images search for Quartiere Coppede or Piazza Mincio.

Not knowing exactly what all you have seen before I'll add the following.

Capuchin Crypt (beneath the Church of Santa Maria della Concezione)
Crypta Balbi and the nearby Largo Argentina
Domus Romane and the nearby Trajan Market
Bathes of Diocletian
Bathes of Caracalla

Palazzo Doria Pamphilj
The Knights of Malta keyhole and the other sites on the Aventine Hill
Ostia Antica

The last time I replied to this sort of question, nearly everything I suggested had already been seen. I'll give it a try.

Near Termini station, Palazzo Massimo alle Terme has one of the world's greatest collections of ancient sculpture, some beautiful floor mosaics, and the wall frescoes from the summer palace of the Empress Livia, wife of Augustus Caesar. There are also household effects, jewelry, coins, and even a mummy of a little girl, buried with her doll. It's not known why this little girl was buried in Egyptian fashion, the only one in her family tomb to be buried this way.

The Basilica of Santa Maria degli Angeli is also very near Termini station. This beautiful basilica was contructed in the ruins of the enormous ancient Baths of Diocletian, to a design of Michelangelo. Another part of the ruins can be seen at the Museum of the Baths of Diocletian. However, not much remains of these baths, which once occupied a great part of the land around Termini station. In fact, the baths ("terme" in Italian) gave the name to the station, and also to Palazzo Massimo alle Terme. If you want to see a beautiful Roman bath, much more intact than the baths of Diocletian, you should visit the Baths of Caracalla.

Also near Termini, the Basilica of Santa Maria Maggiore is one of the oldest in Rome, with wonderful golden mosaics, some from the 5th century.

The Capitoline Museums, above the Roman Forum, also have a great collection of ancient sculpture and other items of historical interest.

Further away, the Palazzo Doria Pamphilj, near the Pantheon, is a Renaissance palazzo still owned by descendents of the powerful Renaissance family. The rooms are furnished in lavish period style, and the family's art collection still hangs on the walls. There is an excellent audio guide, narrated by a member of the family.

The Barberini Gallery, near Trevi Fountain, has a world-class collection of Italian painting, including a rich collection of great masters from late medieval to early modern times.

The Villa Farnesina, in Trastevere, is a Renaissance villa, set in a lovely garden, with beautiful frescoes, some painted by Raphael. The nearby Corsini Gallery has a small but very important collection of paintings. It's a sister gallery of the Barberini Gallery, and one ticket gives you entrance to both, overa period of 10 days.

Tivoli for Villa d'este and Hadrian's Villa?

Thank you!
All places I have never heard of!