The Loggia dei Lanzi, also called the Loggia della Signoria, is a building on a corner of the Piazza della Signoria in Florence, Italy, adjoining the Uffizi Gallery. It consists of wide arches open to the street, three bays wide and one bay deep. The arches rest on clustered pilasters with Corinthian capitals. The wide arches appealed so much to the Florentines, that Michelangelo even proposed that they should be continued all around the Piazza della Signoria.
https://en.wikipedia.org/wiki/Loggia_dei_Lanzi

The Basilica di Santa Croce (Basilica of the Holy Cross) is the principal Franciscan church in Florence, Italy, and a minor basilica of the Roman Catholic Church. It is situated on the Piazza di Santa Croce, about 800 metres south east of the Duomo. The site, when first chosen, was in marshland outside the city walls. It is the burial place of some of the most illustrious Italians, such as Michelangelo, Galileo, Machiavelli, Foscolo, Gentile and Rossini, thus it is known also as the Temple of the Italian Glories (Tempio dell'Itale Glorie).

The Uffizi Gallery (Italian: Galleria degli Uffizi, Italian pronunciation: [ˌɡalleˈria deʎʎi ufˈfittsi]) is a museum in Florence, Italy. It is one of the oldest and most famous art museums of the Western world.

The Palazzo Pitti (Italian pronunciation: [paˈlattso ˈpitti]), in English sometimes called the Pitti Palace, is a vast mainly Renaissance palace in Florence, Italy. It is situated on the south side of the River Arno, a short distance from the Ponte Vecchio. The core of the present palazzo dates from 1458 and was originally the town residence of Luca Pitti, an ambitious Florentine banker.

Twain remarks that the gondola he rode in belonged to the Grand Hotel d' Europe, so I assume that is where he stayed while in Venice. I did not find a listing for this hotel on Google but I did find the Hotel Londra Palace of Venice operating in 1853.

The Bridge of Sighs (Italian: Ponte dei Sospiri, Venetian: Ponte de i Sospiri) is a bridge in Venice, Italy.

The Patriarchal Cathedral Basilica of Saint Mark (officially known in Italian as the Basilica Cattedrale Patriarcale di San Marco and commonly known as Saint Mark's Basilica) is the cathedral church of the Roman Catholic Archdiocese of Venice, northern Italy.

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.