The Colosseum complex (which includes the Roman Forum and Palatine Hill) and the Vatican Museums are two destinations that can have horrendous lines in high season (roughly Easter through the end of October), and even lines over the winter.  “Skip the line” tickets, which can easily be booked online, are highly recommended.  For both the Colosseum and the Vatican Museums, you can book through their official websites:
https://www.coopculture.it/en/poi/colosseum/
https://tickets.museivaticani.va/home