One day for ancient Rome, one for the Vatican. Everything in the centre is walkable, so the plan follows the streets between the sights rather than taxis. Book the Colosseum and the Vatican Museums online before you go: both sell out.
Ancient Rome
08:30
1The ColosseumBook the timed ticket online; it also covers the Forum and the Palatine Hill.Open in Maps
11:00
2The Roman Forum & the Palatine HillThe old city centre in ruins. The view over the Forum from the Palatine is the best one.Open in Maps
14:00
3Lunch, then the PantheonA 2,000-year-old temple; the hole at the top of its dome is nine metres across.Open in Maps
16:30
45Piazza Navona & the Trevi FountainFifteen minutes' walk apart. Trevi is quietest late in the evening.Piazza Navona:Open in MapsTrevi Fountain:Open in Maps
20:00
6Dinner in TrastevereCobbled streets across the river, full of trattorias. Order cacio e pepe.Open in Maps
The Vatican & the Spanish Steps
08:00
7The Vatican Museums & the Sistine ChapelBook ahead and come at opening time; plan three hours.Open in Maps
11:30
8St. Peter's BasilicaFree to enter. Climb the dome (stairs, or a lift part of the way) for the view over the square.Open in Maps
15:00
9The Spanish Steps & Via del CorsoLook down from the top of the steps (sitting on them is fined), then walk down the shopping street.Open in Maps
18:00
A last gelato, then the airportThe Leonardo Express takes 32 minutes from Termini to Fiumicino.
1 Colosseum2 Roman Forum3 Pantheon4 Piazza Navona5 Trevi Fountain6 Trastevere7 Vatican Museums8 St. Peter's Basilica9 Spanish Steps
This is the kind of plan I write. Tell me yours, where, when and who's coming, and I'll check the roads, the stays, the weather and the costs.