Probably Rome's most famous landmark, the Colosseum dates back almost 2000 years, being commissioned by emperor Vespasian in 72AD. It is situated in the centre of Rome and is easily accessible by foot or on the Metro. We would recommend going in the morning, around 8:30, whereby you can avoid the crowds and you can see it in the best light. Book a ticket in advance which includes an audio tour of the Colosseum and access to the forum and Palatine Hill, it'll also allow you to jump the queue!
Once you've absorbed all you can at the Colosseum then wander over to the Roman Forum and explore the numerous ruins that are here, you probably don't need an audio tour for this as it's well documented on signs. Finally, head up the slope to the top of Palatine Hill to see the ruins of the various palaces and also for a great view over the city.
Interesting fact: The word palace stems from the Latin word Palatium which was the name for Palatine Hill.
Best Time: 8:30 AM (First thing in the Morning)