Baiae (in modern Italian Baia), a frazione of the comune of Bacoli in the Campania region of Italy, was a Roman seaside resort on the Bay of Naples. It was said to have been named after Baius, who was supposedly buried there. Baiae was for several hundred years a fashionable resort, especially towards the end of the Roman Republic. Baiae was even more popular than Pompeii, Herculaneum, and Capri with the super-rich, notorious for the hedonistic temptations on offer, and for rumours of scandal and corruption.

Sam and companions spend two days on the island of Ischia while the Quaker City remained in quarantine at Naples, from August 7 to August 9, 1867. Mark Twain Project: Quaker City Itinerary

Museo Civico

The Blue Grotto (Italian: Grotta Azzurra) is a noted sea cave on the coast of the island of Capri, Italy. Sunlight, passing through an underwater cavity and shining through the seawater, creates a blue reflection that illuminates the cavern.
https://en.wikipedia.org/wiki/Blue_Grotto_(Capri)

August 10, 1867, Sam and companions visited the Island of Capri by chartered steamer. Mark Twain Project: Quaker City Itinerary

The Mausoleum of Hadrian, usually known as Castel Sant'Angelo (Italian pronunciation: [kaˈstɛl sanˈtandʒelo]; English: Castle of the Holy Angel), is a towering cylindrical building in Parco Adriano, Rome, Italy. It was initially commissioned by the Roman Emperor Hadrian as a mausoleum for himself and his family. The building was later used by the popes as a fortress and castle, and is now a museum. The Castle was once the tallest building in Rome.
https://en.wikipedia.org/wiki/Castel_Sant%27Angelo

The Vatican Page 234 Bædeker Central Italy and Rome (1869)

The Capuchin Crypt is a small space comprising several tiny chapels located beneath the church of Santa Maria della Concezione dei Cappuccini on the Via Veneto near Piazza Barberini in Rome, Italy. It contains the skeletal remains of 4,000 bodies believed to be Capuchin friars buried by their order. The Catholic order insists that the display is not meant to be macabre, but a silent reminder of the swift passage of life on Earth and our own mortality.
https://en.wikipedia.org/wiki/Capuchin_Crypt

The Borghese Gallery (Italian: Galleria Borghese) is an art gallery in Rome, Italy, housed in the former Villa Borghese Pinciana. It is a building that was from the first integral with its gardens, nowadays considered quite separately by tourists as the Villa Borghese gardens. The Galleria Borghese houses a substantial part of the Borghese collection of paintings, sculpture and antiquities, begun by Cardinal Scipione Borghese, the nephew of Pope Paul V (reign 1605–1621).

S. Maria Rotonda, or the **Pantheon;  Page 153 Bædeker Central Italy and Rome (1869)