Carrie, referencing your previous post, you look to have 2.5 days in Rome, yes? I'd lean heavily towards NOT trying to do a "Rome in a day" tour if at all possible. Specifically, breaking up the one-day itineraries to cover the Vatican Museums/St Peter's on one day, and the Colosseum/Forum/Palatine on another instead will ease the fatigue of being smack in the middle of the densest crowds at Rome's most-visited two attractions.
You didn't mention WHEN your trip will be in that previous post but if it's during the May-Sept. months, dealing with the heat just adds to the exhaustion/discomfort factor.
A morning visit to the Vatican Museums and St Peter's could be combined, after lunch, with an afternoon swing across Ponte Sant' Angelo (with its lovely Bernini-designed angels) to Trevi Fountain, Piazza Navona and Pantheon. A morning visit to the Colosseum/Forum/Palatine could combine with an afternoon look at Basilica San Clemente (2000 layers of history) and old Basilica S. Giovanni in Laterano (the Pope's church as bishop of Rome) or a wander through Jewish Ghetto/ruin of Portico d'Ottavia and Teatro di Marcello and over to Largo Argentina, an excavation of temples and curia where Julius Caesar was assassinated.
The above are just suggestions; arrange your afternoons depending on energy levels, interests and weather. I'm not particularly religious but enjoy many of the churches of Rome for their architecture and artwork PLUS most of them are free.
To add: outdoor attractions such as Trevi Fountain, Piazza Navona , Ponte Sant'Angelo, etc. are also nice to do at night when temps have cooled and the city's fountains and monuments are all lit up. It's very safe to walk after dark; you will be in good company!