If you ever get to Rome, don't miss the Pantheon.
I've visited the Pantheon on two separate trips to Rome. The first time I encountered it, I was just wandering around Rome, getting kind of lost as I often enjoy doing on trips. Then there was this moment of serendipity, and there it was. Moments such as this can make any trip worth the time and expense.
The Pantheon is considered the best preserved monumental building from the Roman Empire, and it is a marvel to enter and experience, perhaps after one visits some of the Roman ruins. Then you can get a better mental image of what Rome may have been like, in terms of architecture, in ancient times.
When I visit very old places, I like to think of who may have stood on the very spot where I'm standing. I like to think of the historical events that happened there. Instead of just busily taking photos that will soon be forgotten, I prefer to live in the moment of being in that place. So it was with the Pantheon.