One of the most beautiful sites in Vietnam is Ha Long Bay. I took a 2 days one night tour to this place. I have seen something simillar in Thailand (Phang Nga and Ang Thong) but not exactly the same. I have visited there a couple of nice caves. I think I was lucky to have good weather this time of year. There are hundreds of mountains coming out of water. In the afternoon I did a bit of kayaking, and before the dinner some diving from the boat (4-5 meters) and swimming. The water was warm but not clear. The bay is marketed as a romantic place. It was definetely worth seeing it.