Next week, I have business in Hanoi, Vietnam. I have only a 1-day break. I want to visit HaLong Bay and enjoy a cruise for one night. Any recommendations about how to get to HaLong Bay from Hanoi and what's the best cruise there?
Recommendations are off topic for this site - you can find loads of reviews on TripAdvisor however. For some advice, I'd say that to some extent you get what you pay for with Halong Bay - cheaper cruses will probably be "party boats" with dorm-style bunks and all drinks and food charged as extras.Getting nice facilities, good food etc costs more. There are several different routes that can be followed as well, so consider that when picking a cruise.
With regards to getting there: The easiest way is probably to simply get a crusie with a transfer from Hanoi included. A great many tour companies run minibuses or even full coaches down the highway between Hanoi and Halong City and this is often included in the cost of the cruise. Most of these minibuses do however stop for 30 minutes or so each way at a "rest station" which will largley be an attempt to sell you statues, paintings and jewelery. If you want to be quicker, then a personal car transfer/taxi will save you this stopping time. It's also possible to get the train to Haiphong, but I'm not sure how long the journey will take to get from Haiphong to Halong.
If you only have one day however, this my be problematic. Between the 3-4 hour journey there (and the same time back) and the cruise itself especially to see much interesting, a one-night cruise will typically have you out of Hanoi for 2 whole days (as the boats are not allowed to move in the dark, there is a limit to how early they can have you back on shore for, also). There are cruises available without an overnight stay - with carefully planned transfers, this may work for you.
An alternative option to investigate if time limitations are a big issue is booking a private cruise - that is chartering a boat just for you (and any other members of your group, if there are any). This will be expensive (compared to standard tours), but you can make the times work for you - which may be important iof you need to get back to Hanoi for work or flights.
If you want to travel around Halong with 1 day, i think it is impossiable beacuse you spend 4 hrs to Halong from Hanoi. Furthermore, the best way to explore Halong is cruise which you should overnight. Vietnam Tour Booking is travel agency which i booked tour for my family last year with Halong Charming Cruise. It's quite reasonable and good services.Beisdes you can find more at Tripadvisor and Lonelyplanet which can provide reviews and suitable itinerary for you Happy travels!
You leave early in the morning and get to Ha Long (180 km) in 3 hours and a half or so, by 12 noon. Than you are offered a local lunch on the boat while visiting the outdoors and later the caves. You may be offered the chance of kayaking in some dedicated area, or visit Titov Island or some other selected place within the bay. The coaches get back to the capital town after dinner time but not too late at night.
Surprisingly this wasn't too tiring: consider that commercial speed of Vietnamese buses is very low. This is why back in the days (January 2019) I wanted to try this excursion while, for example, I considered a one-day trip to Sa Pa (about 320 km away) not feasible.
At the end of the game, I recommend such an organization if you don't have too much time, although I imagine that dedicating two days to this cause would make the experience even better and deeper.
This is certainly possible but rather tight. There are independent ways of getting to Ha Long from Hanoi but since the cruise departures are rather strict, there is a very high probability of missing the boat. Agencies know that, so most will sell tours with transfers to and from your hotel.
There are huge number of tours there. You can pay in advance before getting into Vietnam which will give you a high price but a predictable experience. On the other hand, if you book once in Vietnam you will likely end up on the same boat and pay roughly half.
What I would recommend is you take two or three hours when you just arrived into Vietnam to visit agencies and buy your tour from there. In Hanoi, there are hundreds of these agencies and many of them are within proximity of each other, so you can quickly shop around. On the other hand, if you are pressed for time and want to get a better experience without minding a higher price, most hotels can book you a tour themselves.