Many guidebooks, in addition to Rick's, include self-guided walking tours, and he's provided some audio tours which can be dowloaded for free:
in Rome, you might consider a tour which covers the Colosseum, Roman Forum and Palatine. While the Colosseum is pretty easily understood with just a little advance reading or an audioguide, the other two are much more complex. Most combo tours last around 3 hours or so. I'll say the same for the Vatican Museums as they are VAST and can benefit from the direction of an experienced guide. If they have resumed them at the time of your trip (there's some confusion around whether this itinerary is currently available or not) I'd recommend a tour which includes early entrance to the museums + St Peter's basilica.
Recommendations from RS posters over the years:
Walks of Italy:
The Roman Guy:
The Vatican offers its own tours of the museums: options and tickets here:
You'd likely want the "Guided Tours for Individuals - Vatican Museums and Sistine Chapel". They don't offer one which includes the basilica at this time.
I don't personally think there's anything in Florence which needs a human guide, although advance reading is highly recommended. Ostia Antica: it's possible they'll resume guide services at the site by the time you go but here's printed guide you can download in advance for seeing the site on your own, if interested:
I would refer to the website Christine provided in one of your earlier posts for any visiting news updates.
Just a start !!!