01 St Mark's Basilica & the Doge's Palace
From €28 for reserved entry, and the two are almost always sold together because they share a square and a queue. The basilica is fifteen minutes and free to enter without the loggia and the Pala d'Oro, both of which are the reason to pay. The Doge's Palace is the longer visit; the Secret Itineraries route is the one that rates highest and the one that sells out.
02 Murano & Burano
Boat trips from €15, and the ones to avoid are those that make the glass factory the point — the demonstration is ten minutes and the shop is thirty. The versions that rate well give Burano an hour, which is what it needs. The public vaporetto does the same route for less if you are happy to navigate it yourself.
03 The gondola
From €37 per person shared, and the fare is fixed by the city rather than the operator: what you are choosing is the route and whether six strangers are aboard. Shared rides leaving from the smaller canals rate far better than those queuing at the Rialto, for the obvious reason. Thirty minutes is the standard; that is long enough.