Ha Long Bay day trips from Hanoi
Out and back in one day. About thirteen hours door to door, of which six are on the water and seven in a van.
The day trip exists because plenty of itineraries have no spare night, and it does deliver the bay. You will see the karst, visit a cave, and usually get a swimming or kayaking stop with lunch on board.
What it cannot give you is the evening. The boats turn for the pier in the early afternoon, and everything people describe as the best part of Ha Long happens after that, when the day fleet has gone and the water goes still.
Book it when the calendar leaves no choice. If you can find one spare night, compare the day-trip price against an overnight, because once you add another night of Hanoi accommodation the overnight is often the cheaper trip as well as the better one.