If you must have a Bellini, have it at Harry's Bar where it was invented. Go during the cocktail hour when they serve their tiny croque monsieur sandwiches to go with it (or at least they did many years ago...).

At Florian's, again, during the predinner period, you would do better to order a glass of wine or, if you must have bubbly, a glass of prosecco (which is what Harry's Bar adds peach juice to in order to make a Bellini). They will give you nibblies to go with it; they did as recently as 2003.

About the Scuola di San Rocco: I would agree with Ssachida that it should not be missed, but I would point out that there are two floors, not only one, of paintings by Tintoretto.

Is the small island, San Giorgio Maggiori, the same place where you go to hear the monk's chant? Thanks!