Ha Long Bay
You simply can't visit Vietnam without seeing Ha Long Bay.
While parts of it can be overrun by tourists, pick the right operator and take off for a one or two night tour and you will find yourself in some of the most peaceful scenery in all of Asia. Declared as a UNESCO World Heritage Site in 1994, the bay is located around 3-4 hours driving time from Hanoi, and is home to over 1,600 limestone pillars.
Paradise Cruises offer nine exquisitely-furnished ships and a range of one- or two-day options.