There is just too much to see when in Rome. We saw an incredible amount in 4 days. The main tip I can give you is START EARLY!!!
If you head to the Vatican (early!) first get in line for St. Peter’s Basilica because by noon the line is impossible. We spent maybe 3-4 hours at the Basilica. We paid the extra euros (7,00) to take the elevator halfway up the cupola and it was well worth it because after that we climbed a million more stairs to the top. If you are claustrophobic then do not venture to the top, the spiral staircase continues up, the walls narrowing along the way.
Next, get in the long line for the Vatican Museums (Sistine Chapel). The line can be long but it goes fairly fast. I think we waited in line for at most maybe 1 hour (we got in line at about 12:30/1pm). The tickets are 16 euros if you don’t buy them through the internet. You can reserve them online, the wait is shorter but you will still have to wait in a different line.
Don’t miss out. This is the best part of the trip.
Set aside almost a whole day for this visit. Buy your tickets for the Colosseum and the Forum at any entrance for the Forum because the Colosseum line is always way too long. The tickets are good for 2 days but only one entrance (meaning you can go to the Forum one day and the Colosseum the 2nd or vice versa) If you pay for a Forum Tour, then visit the Palatine Hill beforehand because it is not included in the tour but it is one of the best parts of the Forum. And if you want to get in on the Colosseum underground and upper tier tour then book in advance because the tour tickets sell out fast! Also, if you don’t book the tour then you don’t have access to the underground or upper tier of the Coliseum. I wish I could tell you if it is worth it but it was sold out, yet I’m sure it is worth it because even the regular entry experience is quite impressive.
And there is so much more to see! Definitely don’t skip the Pantheon.