Rome is a city that has been consuming tourists for two thousand years and has developed sophisticated systems for extracting money from people who don't know what they're doing. This is not a criticism — it's just accurate. Here is how to not be those people.
Geography first
Rome is large. The Colosseum is on the southeast side of the historic center. The Spanish Steps are on the northwest. They are not near each other. Trastevere is across the river. Plan accordingly — walking everything will exhaust you.
The Vatican trap
Every restaurant within 400 meters of the Vatican is a tourist trap. The food will be mediocre. The prices will be insulting. You will feel cheated. Walk 10 minutes in any direction and find somewhere that has a menu in Italian only.
Skip the queue
Book the Colosseum online, at minimum 2–3 days in advance. The line without a reservation can be 2+ hours. With a reservation, you walk in. This is not optional information — it is critical.