Ha Long Bay cruises, 2 days and 1 night

Two days and one night is what most boats are built around, and unless you have a specific reason to stay longer it is the trip to book.

The shape is consistent across operators. A van collects you from Hanoi around eight, reaches the pier between half eleven and half twelve, and lunch is served as the boat leaves. The afternoon is a cave and a kayak or swimming stop. Evening is on deck. The next morning brings an early activity, brunch while sailing back, and disembarkation before noon, putting you in Hanoi mid-afternoon.

Be honest with yourself about the arithmetic: of roughly twenty-eight hours door to door, about seven are in a vehicle. The night on the water is the point, and it is worth it, but nobody tells you about the road in advance.