San Carlo Theatre is an old and large theatre in Naples, which is must visit if you're an Opera fan. The Opera House is the oldest theatre in Europe and is truly a beautiful gem in Naples, and can be visited only by a tour guide - this way you can learn all about the history of this old theatre.
If you're lucky, you can also witness a performance here. The entire building, stage and even the acoustics is excellent. There's also a very reasonabl priced cafe below.