If you've visited Hanoi - there is a good chance you've visited or at least heard of Halong Bay. It's a really beautiful place.
When Rich and I visited Vietnam we spent a couple of nights on a tour in Halong Bay - cruising around on a fancy wooden boat and swimming, checking out some caves and enjoying the view.
Some people will tell you when you visit South East Asia you need to dress comfortably for the weather and wear loose, comfortable clothing.
I think when you travel in South East Asia you must make every effort to look as adorable as you possibly can and most importantly, if you're going to be on a boat you MUST wear something with an anchor on it.