Ideas for Rome that don't include the regular tourist sights

I enjoyed the Botanical Garden in Trastevere this past March. It was a nice escape from the crowds.

https://web.uniroma1.it/ortobotanico/en

I liked the Palazzo Barberini Museum. It was very uncrowded. Next time I'd like to see the Doria Pamphilj Gallery.

One of my favorite places outside of Rome was Ostia Antica. It's about a 30 minute train ride from Rome and the entire setting is a delight. Ostia is a large archaeological site that is the location of the harbour city of ancient Rome. It was covered in silt from the Tiber River over 1000 years ago, and discovered in the 1800's, so it is relatively intact. Some of the ruins go back to the 4th century BC. There are many ancient buildings, magnificent frescoes and beautiful mosaics.

There is a park nearby where you can bring a picnic lunch and sit and enjoy the beautiful setting also. IThe link above is the official website, but this site is worth a visit as it has a lot of information about the archeology and the historic town. https://www.ostia-antica.org

We stumbled across this in April but didn't have time. It's on the list for next time. Honestly have no idea if it's a popular site. There is a stadium that was originally built where Piazza Navona is now. https://stadiodomiziano.com/

Enjoyed San Clemente as well 10 years ago. There is a pagan church underneath the current church. https://basilicasanclemente.com/eng/

You could take a tour of Palazzo Venezia which is smack in the middle of the city but its interior is often overlooked. It's where Mussolini had his headquarters but also has a long history before that. What used to be his parking lot is now a lovely garden. The garden itself is free to access. The tour only takes an hour or so.

All awesome ideas. Is the Jewish Ghetto and along the river a good idea as well? Tiber Island?

Park of the aqueducts:
https://www.parcodegliacquedotti.it/

Catacombs
https://www.rome.net/catacombs-rome

Tiber Island is mainly occupied by a hospital but walking along the river and through the Ghetto is enjoyable. I wouldn't consider it off the beaten path though.

Check out the Capuchin Crypt and ponder your own doom

If you visit Tiber Island, in particular the Fatebenefratelli Hospital, make sure you don't catch the disease called Morbo di K (K Syndrome or also K Disease), a very infectious disease the hospital is famous for.

I really enjoyed the Domus Aurea where they offer a 1 hour tour. Was surprised how easy it was to get tickets and it’s around the corner from the Colosseum

I would recommend a stroll to Trastevere, whether to eat or just wander. Really enjoyed the neighborhood although there were a few streets very packed in the evenings given all the restaurants.

Have you already seen these ideas?

https://community.ricksteves.com/travel-forum/italy/unusual-ideas-for-rome

I know you said no museums, which I respect, but this idea might be interesting either to you or others reading the discussion: The "Museo Historico de Liberazione", for anyone who has any interest in World War II history (something Rome doesn't otherwise feature). Small, interesting, in the old Gestapo headquarters, not far from San Giovanni Laterano. I admit I have kind of a proprietary interest in this one, because some years ago I found it in a very obscure guidebook (it wasn't in any of the normal ones), visited it, and recommended it in feedback to the RS Rome guidebook, and lo and behold, there it was in the next edition. I have no idea if that was direct cause and effect, but I suspect that it was, that the RS people checked it out on their next update visit and found it worth a mention.

Have to agree with Mardee about Ostia Antica. In many ways it's better than Pompeii or Herculaneum. A lot less crowded and a much easier day trip.

Slate, I visited the museum you described while I lived in Rome, and found it extremely moving. The graffiti on the cell walls was poignant, the whole place was overseen by one guard, and it was just an intimate experience. I can't recall if I found it in the RS guide or some other way, but I'm definitely glad I went. Thank you for your post.

San Clemente is very interesting.
The Basilica containing the statue of Moses, and the chains of St Peter.

https://www.lateranensi.org/sanpietroinvincoli/

There is also an ArtDeco or Art Nouveau neighbourhood that I have yet to get to in my trips to Rome.
I’m sorry, I don’t remember the name of it.

Quartiere Coppede is the Art Nouveau neighborhood. It is very attractive.

I'm a big fan of mosaics, so I especially liked these churches: Santa Prassesde (spelling varies), Santa Pudenzia and Santa Cecilia in Trastevere. The first two are near Santa Maria Maggiore, not too far from Termini Station,

I recommend Santo Stefano Rotondo, a church not far from the Colosseum. It has 33-ish frescos depicting Christian martyrs. It's peaceful, easy to get to on foot from the Colosseum, and, I think, interesting. The frescos were created to remind Jesuit missionaries what fates might await them as they spread the Gospel. Of this place, Charles Dickens wrote, “...such a panorama of horror and butchery no man could imagine in his sleep, though he were to eat a whole pig raw, for supper.” Okay--maybe that doesn't sound inviting, but it really is an unusual church to visit.