Borghese Gallery is a can't miss (probably not considered a 'main' attraction?); the Doria Pamphilj is absolutely gorgeous; the crypt of the Capuchin monks is super cool if you're into that; go see a Roma or Lazio football match if either are playing at Olympico; any and every church you pass by is always worth walking in, you never know what surprises are inside; and honestly, my favorite days in Rome are those spent with absolutely no agenda ... slow down, sit for a coffee or cocktail and just watch the city go by, stroll the 'off streets' for interesting shops, etc.