This museum is probably the best one that you've never heard of (unfortunately). It has more Greco-Roman antiquities than I've seen in more well-known sites and its various treasures will keep you occupied while escaping summer's heat. Even if you haven't made it to Pompeii or Herculaneum yet (albeit not far from Napoli), the mosaics and statues will give you a good history lesson. And who said that a museum can't have a sense of humor? Make sure to check its "Gabinetto Segreto" for even more insight into Napoli's naughty past.