Rome is an amazing city! There's so much to see and do, you can't possibly see it all in one trip (no matter how long your trip). In this video I outline some of my favorite can't miss sights to help you get an idea of what to see on your trip to Rome.
Vatican Museum and St. Peter's Basilica - Every trip to Rome should include this incredible museum and church. While here make sure to climb to the top of the dome for some amazing views of Rome. When you finish here, you can walk down to Castel Sant'Angelo.
Colosseum, Roman Forum and Palatine Hill - These three sights are all on the same ticket. You can save a lot of time by purchasing your tickets in advance or getting them at either the Palatine Hill entrance or the Roman Forum entrance. Join a walking tour to make the ruins come to life and to go in the hypogeum (Underground) level of the Colosseum.
Walk Through the Heart of Rome - This is a wonderful way to see a lot of the major and free sights of Rome. I love to do this walk around 6am (or sunrise) to see the sights without crazy crowds of people. Start at the Spanish Steps and (in this order) go to Trevi Fountain, Piazza Colonna, Pantheon, Piazza Navona and Campo de' Fiori (and enjoy the morning market). Although you can do this walk at any time of day, I love the early morning.
Piazza del Popolo - A very large beautiful piazza great for enjoying people watching and hanging out. After a nice visit here take a walk down Via del Corso (pedestrian street) and do some shopping.
Borghese Gallery and Gardens - This beautiful gallery featuring statues and paintings by Bernini and Caravaggio (and many more). You MUST reserve your ticket and visiting time in advance. Before or after you visit the gallery, take some time to enjoy the grand Borghese gardens (They even have a zoo).
Capitoline Hill and Museum - Climb up the grand staircase designed by Michelangelo to get to one of the best museums in Rome. Also, from this hill, you can get a great view over the Roman Forum.
Evening "Projection Light Shows" in various Imperial Forums - Take a walk through time with a guided audio tour inside one of the forums (it used to be closed to the public) and as you go it will show you with projection lights what things were like back in ancient Rome. You can also watch another show in a different forum while sitting on a small set of bleachers.
Visit a few Churches - There are over 900 churches in Rome, so don't feel like you need to go inside them all. A few of my personal favorites are... San Giovanni, Santa Maria Maggiore, Santa Maria degli Angeli, San Clamente and San Pressede. Plan out a day to visit these or other churches for a nice walk through Rome.
St. Paul Outside the Wall - This is a sight many people miss, but it is incredible. As you can see in the video, they were singing in the church that day. To get to St. Paul's you just take the Metro (Subway) B Line (towards Laurentina) and get off at the "Basilica di San Paolo" stop.
Catacombs etc. - Some additional sights I suggest seeing if you have the time are... The Catacombs (preists guide the tours), Baths of Caracalla (grand bath house), Tivoli (small town outside Rome where you can visit Hadrian's Villa and/or Villa d'Este and it's hundreds of fountains), and last, but not least, Ostia Antica (considered a mini Pompeii).
This is just the tip of the iceburg for sights in Rome, but it's a good start and more than enough for a trip of 3-7 days. Make sure you plan your major sights in advance so you don't waste time waiting in line. Don't plan too much each day and make sure you maximize your time by planning to see sights that are near each other in the same day if possible.
(NOTE: Want Our Rome Cheat Sheet? Copy these done-for-you itineraries to make sure you see all of these amazing sights the right way. Get them here.)