I'd say Halong Bay is slightly similar to Krabi but no way is it "much the same as..." I consider Halong a MUST for anybody in the Hanoi region, we did the tour in bad weather (January) but it was still stunning. I remember standing on deck of our junk looking out across that remarkable scenery with my MP3 player turned up full blast and listening to some outstanding songs.
We also went to a performance at the Hanoi Opera House. Fortunately there was a special performance on at the time we were there, so I took my wife to dinner then to the opera. It was one of the best evenings we've spent together.

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