If you’re looking for the experience of strolling through gigantic Roman ruins, without the crowds, go see the Baths of Caracalla. While everyone else is shoulder-to-shoulder trying to get into the Colosseum and Roman Forum, walk a mile south of the Colosseum and see these huge Roman ruins without huge crowds (there will be people around, but the atmosphere is a lot more relaxed... probably just as it was when this giant Roman aquatic / rec center / library / hangout place was in operation):