Even though Rome is one of the great big capital cities of the world, many people don't realize that the historical center can be covered on foot, and in just one day, you'll cover quite a bit. Armed with a map or a smartphone and your camera (or that smartphone, again), follow this itinerary and you'll knock off most of the main attractions. And that means only one thing: more free time for pizza and gelato!
Colosseum and Roman Forum
It's always best getting to the Colosseum early. A sure way to beat the lines is to buy an audio guide. Your ticket gives you access to the Roman Forum, too. Spend the morning taking Ancient Rome in. A great lunch spot nearby is Ai Tre Scalini in the Monti neighborhood.
Trevi Fountain
Next up, the famous fifteenth-century Baroque fountain that is the most famous in the world. Remember that it's two coins: one for a wish and the second to guarantee your return to Rome. 
The Trevi Fountain — Photo courtesy of HeartRome
Spanish Steps
A quick side trip from the Trevi is the Spanish Steps. This staircase is the largest in Europe, and you should climb to the top - Trinita dei Monti - for an amazing view of the city.
Via dei Condotti
The street leading directly off the square is Via dei Condotti - Rome's designer strip. Take this through to Via del Corso, your non-designer and regular strip. Just to get your bearings here, if you were to turn right, you would reach Piazza del Popolo. If you look left, in the distance you'll see the big "wedding cake" (a monument to Vittorio Emanuele II). Today, you're turning left.
Hadrian's Temple and the Pantheon
Walk up the corso (as the Romans like to call it) until you reach Via di Pietra and turn right. Just keep walking until you reach Piazza della Rotonda where the Pantheon sits. On your way, you'll pass a series of columns that are the remains of the Roman emperor Hadrian's temple on your left. The Pantheon is free to enter, and at 2000 plus years old, it remains the largest and most complete standing structure of Ancient Rome.
Piazza Navona
As you walk out of the Pantheon, walk straight up the square and then left on to Salita de Crescenzi and follow the road for a few minutes until you reach Piazza Navona. What once was a former stadium now boasts the city's best example of Bernini's work - the fountain that is the square's centrepiece also features an Egyptian obelisk. Piazza Navona is super pretty for a leisurely stroll and is full of street artists and painters selling their wares. 
Piazza Navona — Photo courtesy of Eating Italy Food Tours
The Vatican by night
The Eternal City at night is a bit of a show off, actually! Beautifully lit, its monuments just shine after dusk. In the evening, why not explore the Prati district, which is near the Vatican? For the best at-feet-level view of St. Peter's dome and square, walk past Castel Sant' Angelo (the former fortress of the Pope), cross the Tiber River and head up Via della Concilazione. Take some pics and then walk away - the Vatican Museums are a day in themselves.
To the right of St. Peter's, you'll reach Piazza del Risorgimento and the European-style boulevard streets like Via Cola di Rienzo. Shops remain open until 8 p.m. and bars around here serve aperitivo beginning at 6 p.m. For an authentic Roman feed, try Hostaria Dino e Toni (And trust Dino with all the ordering.).
Sit down and look at the picture-perfect postcard moments you've created. Relax and have some vino. Your feet will thank you for the much needed rest.