There's so much to see in Rome! As already suggested, I'd as many days as you can so you can enjoy a nice selection of her treasures without having to do that at a dead run!
Colosseum/Forum/Palatine have already been mentioned, and I'll recommend booking a tour that includes all 3. While the Colosseum is easy to get one's head around - its plan is still the plan for many sports arenas today - the Forum and Palatine are more complex and benefit from a guide. If really NOT into shuffling about in ruins, then maybe just the exterior of the Colosseum is enough. Some companies to look at which provide tours that include all three:
https://www.walksofitaly.com
https://www.througheternity.com
https://theromanguy.com
San Clemente has also been mentioned: I'll concur although unfortunately it's under restoration so while open, hard to say how much is under scaffolding in November. https://www.basilicasanclemente.com/eng/
Trevi is just a walk-by: do it any time of day although early morning and later evening is preferable due to lighter crowds. To get up close and personal with the lower basin involves a 2 € ticket. You do not have to purchase in advance, and you can still see the thing for free outside of posted visiting hours. Don't go Monday mornings when they clean it.
https://fontanaditrevi.roma.it/en
I'll add Galleria Borghese to the list. This is a wonderful collection of art in a structure that was specifically designed and decorated to show it off. Unlike the Vatican Museum's VAST collection, this one only takes 2 hours to enjoy...and in fact sells tickets for mandatory, 2-hour visiting slots: the galleries are cleared between each of those 2-hour slots. Do not wait too long to book your tickets when they become available for November: this one regularly sells out.
https://www.tosc.it/en/artist/galleria-borghese/
Rome's Cathedral? That would be the Archbasilica of Saint John Lateran. While not as big as St Peter's, its history stretches back much further (it's the oldest public church in Rome) and it houses the cathedra (throne) of the Bishop of Rome...which just so happens to be the Pope. It's well worth a visit, and has a baptistry that was not just the first in Rome but was the model for other ancient octagonal structures built for the same purpose.
https://www.basilicasangiovanni.va/en.html
https://en.wikipedia.org/wiki/Archbasilica_of_Saint_John_Lateran
Churches...oh my. So many are very old, very interesting, ridiculously glorious, or all three. And I'm not even very religious. Most are also free so are super bargains for tight budgets.
Very cool piazzas: Navona, del Popolo, Campo de' Fiori, di Spagna, and more.
Anyway, a guide book is going to be really helpful as far as picking and choosing what piques your interest. HOHO bus? I wouldn't personally recommend it as too many of Rome's streets are too narrow to accommodate them, traffic can be fierce, wet weather could have you trying to see out raindrop-covered windows, etc. Rome is a walking city when at all possible; really the very best way to experience this particular historic center. There are public buses, and a somewhat limited metro system that can be useful depending on where you want to go. There are passes of differing values (e.g. 1 day; 48-hour; 72-hour; etc.) that can be a convenience if you intend to take transit enough to make good on the price. What's nice about the passes is that you only have to validate them the first time you use them.
https://www.atac.roma.it/en/tickets-and-passes
There's also tap-and-go and contactless credit card payment; whatever you're most comfortable with. I'm sort of a pass girl as I prefer not to have a phone or credit card in my hand on crowded conveyances or in crowded placed unless I absolutely have to. Whatever works for you!