As gorgeous as Rome is, unless you step out at 6am or midnight, you can't really avoid the crowds here. But do a bit extra research, and it could really turn out to be the trip of a lifetime in every sense. I am personally that sort who goes to museums, art galleries and historical places to immerse myself in the beauty and history around me. I like to take my time to look around and can't tolerate people elbowing me in the face.
Rome is a place which has a high tourist season nearly all around the year and the only way to avoid queuing up is to book your tickets in advance. I went to Rome in August which is not exactly recommended for several reasons, mainly the heat and the insane amount of crowds, however, I used the below listed websites / companies to get direct entries to some of the most crowded places including the Sistine Chapel and Colosseum.
Colosseum, Palantine Hill & Roman Forum
These 3 are located in the same complex and 1 ticket gives you entry to all 3. The Roma Pass can be a good option to skip the queues, however, if you are planning to book the tickets separately, book them online on Coop Culture. The tickets are sent over to you via email with a bar code which is then used to scan at the entry gates.
Want to avoid crowds? Go closer to the closing times, as the crowds start to thin out and trust me, observing the sunset from the deck at Palantine hill is a breath-taking experience. We were there at about 7pm (closing time 7:15pm), and the observation deck was nearly empty! Great for those selfies and photo ops.
Sistine Chapel, Vatican Museums and St. Peter's Basilica
Although the entry to St. Peter's itself is free, the entry to the Vatican Museums and Sistine Chapel is paid. This is the official website of the Vatican to book the tickets and this is also probably the cheapest you can get the tickets at.
I wanted to have the most personal experience possible. The museums and the Sistine Chapel between themselves are home to over 20th centuries of art created by mankind and this was my chance to get as intimate as I could with this treasure.
I chose to book my visit with Walks of Italy. The Pristine Sistine is possibly their most pricey package, however, it is also every bit worth it. The gates at the Vatican open to the public at 9:00am, however, with this tour, you get in an 8:00am, which is an hour earlier than the general public. Living up to its name, the guide will take you directly into the Sistine Chapel which is at its most peaceful at this hour, leaving you to wander among Michelangelo's genius by yourself.
Final words: Worth every penny I paid, this is a highly recommended way to experience the Vatican should you be after an exclusive experience.
As its stands with other tourist attractions, the crowds at St. Peter's square start to thin out by 5:30pm and it will be worth the walk to see the St. Peter's Basilica in the dying light of the day and the walk over to the Castel Sant'Angelo for a gorgeous sunset over the Tiber.
The Pantheon, Piazza Navona and Fontana Trevi
One thing which has always worked for me is to divide my day into slots and avoid peak hours when the crowds are at their suffocating best.
The free and open to all attractions like Trevi and Piazza Navona are best visited between 9am to 11am. The Pantheon has entry hours (varies as per the seaon, it can be checked here), and is best avoided between 4pm to 6pm, not just for the crowds inside, but also outside, as it is a very busy square, dotted with café's restaurants and horses (!).
Cheeky bonus tip :)
We made a day trip to Florence and the best way to get into Uffizi and the Galleria dell'Accademia (home of The David) is to pre-book you tickets here. It is their official website, and you can collect your ticket for both the places from the ticketing office at Uffizi. Uffizi is best seen closer to the opening times and the Galleria dell'Accademia is best visited between 1:00pm to 2:00pm when it is lunch time and crowds are considerably lesser.
Disclaimer: Places, websites and tours reviewed in this post have been independently reviewed and are not paid reviews. I am, however, happy to be contacted for collaborations, please see my profile page for more information :)