The Triumphal Quadriga or Horses of St Mark's is a set of bronze statues of four horses, originally part of a monument depicting a quadriga (a four-horse carriage used for chariot racing) The horses were set into the facade of St Mark's Basilica in Venice, northern Italy after the sack of Constantinople in 1204. They remained there until looted by Napoleon in 1797 but were returned in 1815. The quadriga have been removed from the facade and place in the interior of St. Mark's for conservation purposes.

The Lion of Venice is an ancient bronze winged lion sculpture in the Piazzetta di San Marco (St Mark's Square) of Venice, Italy, which came to symbolize the city — as well as one of its patron saints, St Mark — after its arrival there in the 12th century.

The Palazzo Mocenigo detto "il Nero" is a palazzo on the Grand Canal in the sestiere of San Marco, Venice, Italy. Other Palazzi Mocenigo on each side include the Palazzo Mocenigo Casa Nuova and the Palazzo Mocenigo Casa Vecchia. The palazzo is located between the Rialto Bridge and St Mark's Square. It was occupied by the poet English Lord Byron (1788–1824) when he lived in Venice.

The Basilica di Santa Maria Gloriosa dei Frari, usually just called the Frari, is a church in Venice, northern Italy. One of the greatest churches in the city, it has the status of a minor basilica. It stands on the Campo dei Frari at the heart of the San Polo district. The church is dedicated to the Assumption (Italian: Assunzione della Beata Virgine).
https://en.wikipedia.org/wiki/Santa_Maria_Gloriosa_dei_Frari

Sam and companions merely passed through by train on their way to Venice, July 20, 1867.
Mark Twain Project: Quaker City Itinerary

Lake Como (Lago di Como in Italian, also known as Lario, after the Latin name of the lake; Lach de Comm in Insubric; Latin: Larius Lacus) is a lake of glacial origin in Lombardy, Italy. It has an area of 146 km², making it the third largest lake in Italy, after Lake Garda and Lake Maggiore. At over 400 m (1320 ft) deep, it is one of the deepest lakes in Europe, and the bottom of the lake is more than 200 metres (656 ft) below sea-level.

Twain made a day-trip from Como to Chiasso in nearby Switzerland, not mentioned in Innocents Abroad (Mark Twain A to Z)