Secret Rome, the ultimate guide
How to live Rome as a local
The itinerary I suggest goes all around the historic center, crossing different district: Campo Marzio, Sant'Eustachio, Regola, Testaccio, Monti, Pigneto, Trieste. A golden circle, as I like to describe it. And if you don't know some of the places I mentioned, don't be scared...it's an adventure after all!
Forget about Vatican City, Trevi Fountain and Colosseum (at least these won't be the focus of the itinerary) and immerse yourself into the most authentic secrets of the Eternal City.
Just one tip: put a couple of comfortable shoes on, because yes, you are going to move by public trasportation too but a good part of the route will be covered on foot :)
I also recommend to plan this itinerary in a weekday, so you'll find less people around. Please note all the things I mention here are free of charge, no entrance fee needed!