Ha Long Bay day cruises
No overnight, back in Hanoi the same evening. It is a long day for a short time on the water, and whether that trade works depends entirely on your schedule.
A day cruise means roughly seven hours in a vehicle and about six on the water. Pickup in Hanoi is early, usually between seven and eight, and you are back in the Old Quarter somewhere between seven and nine in the evening depending on traffic.
In those six hours you will typically see one cave, one swimming or kayaking stop, and lunch on board. What you miss is the part most people remember: the bay after the day boats leave, when the water goes still and the karst turns from grey to gold. That happens after five, and a day cruise is already heading back by then.
Take a day cruise if your itinerary genuinely has no spare night, or if someone in your party cannot sleep on a boat. Otherwise the overnight is the better trip, and often costs less than a day cruise plus another night of Hanoi accommodation.