Jay Y.
Yelp
You know that classic photo of a distant church in a lagoon with gondolas in front? That photo that always appears in travel websites? That church is the little-known S. Giorgio Maggiore!
Located on an isolated island even from the nearby Giudecca, the only way you can reach this island is by private boat or the infrequent vaporetto route 2, so this island church is extraordinarily tranquil even when Piazza San Marco across the lagoon is crowded.
While the church itself may be not worth the cross-lagoon ride, the bell tower view is, especially during sunset! For €6 (€4 reduced), you can access the top of the church bell tower via a modern elevator and see the view of the Venetian skyline that is perhaps even better than from the more famous St. Mark's Campanile!
But even if you do not intend to pay for a bell tower view, the quiet bank where you can see St. Mark Square from a distant yet clear view without the crowd.
Because of the infrequent vaporetto route, I recommend coming here only if you have plenty of time to afford getting stranded on this isolated church for a bit. I would not place this destination as a must-go, but its peace and quiet is certainly worth a ride if you can afford the time!
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tl;dr version:
1) Isolated island across from the bustling St. Mark's Square
2) Reachable only by few boats, so not recommended if you are short on time
3) Tranquil refuge from the bustle with a terrific skyline view on the bell tower