There are so many things to do in Rome during summer. Many free events and many open air events. The opera at Terme di Caracalla is absolutely amazing though. It’s not free, but it’s definitely worth it.
Terme di Caracalla is an old Roman Baths (Also known as Baths of Caracalla). And they build the stage right in the middle of it. What you may lose in acoustics you gain in atmosphere.
The opera “Carmen”.
And if this wasn’t amazing enough, they also do video projections on the ruins!
The Nabucco Opera
I went two weeks in a row to see both Carmen and Nabucco, and even though of the Carmen songs is my absolutly favourite of all times “Habanera”, Nabucco was wayyy more sophisticated, three and a half hours of opera just flew by!