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!