Teatro Bellini is a restored 19th-century theatre just north of Piazza Bellini, named for the composer Vincenzo Bellini. Built in 1878, it slipped into decline in the 20th century before reopening as one of Naples' active cultural venues.
It is most worthwhile if you are attending a performance, but the building also makes sense as part of a Centro Storico walk around Piazza Bellini, the archaeological museum, and the dense streets of the old Greek-Roman city.