San Carlo Theatre is a historic opera house in Naples and one of the most prestigious in the world. It is located near the bustling Piazza del Plebiscito, the oldest continuously operating opera house in Europe. You can take a guided tour of the theatre to learn about its history, architecture, and backstage workings. The tour includes access to the stage, auditorium, and sometimes areas like the rehearsal rooms. Its museum showcases costumes, props, photographs, and documents.
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.