There is so much to see and do in Rome that is virtually impossible to list everything. Here are some that should not be missed.
Roman Forum and Palatine Hill - The remains of the ancient center of Rome. If you get the opportunity, take a night tour of the Forum and surrounding areas.
Coliseum - Where the gladiators used to fight, often to their death.
The Pantheon - famous for its unsupported dome (and the hole at the top of this dome), it was originally a temple to Roman gods. Now, it's the final resting place of famous names such as the painter Raphael.
The Spanish Steps - near the main shopping district, these are now mainly a hangout for locals and tourists, especially in the evening.
The Trevi Fountain - the largest fountain in Rome. Make sure to stand with your back to the fountain and throw a coin over your shoulder. It supposedly ensures a return visit.
The Vatican, the Sistine Chapel and St. Peter's Basilica - to avoid lines which can sometimes be several hours long, buy tickets in advance. Make sure to have your knees and shoulders covered on your visit, or you will not be let in. On the last Sunday of every month, entrance is free.
Piazza Navona - this beautiful square filled with cafes and restaurants and the fountain of Neptune in its center is the perfect place to relax after a busy day of sightseeing.
The Catacombs - along the Old Appian Way, the tombs of Christians and pagans line the outside of the walls; burials were not allowed inside the walls. The best way to see the Catacombs is with a group or private tour guide, as they can be dark and somewhat confusing.