I was just in Rome. It was extremely crowded, so make sure you have tickets and reservations for the Colosseo, Vatican Museums, and Borghese Gallery. Summer must be unbearable.

Pro tip - don't enter the Roman Forum directly across from the Colosseo; the line is crazy. Walk up to the right along Via dei Fori Imperiali to the East entrance - or even better - along Via di San Gregorio to the south end of Palatino Hill, which is least crowded.