I agree that June/July is pretty bad weather there - very hot/very wet/very humid. Halong Bay is also not the best place to swim - water is quite polluted, and the air is often smoggy.

Personally, I love the city of Hanoi for the chaos of it's city life ( a great walking/exploring city ). Sapa is beautiful/colorful and even the train trip up there is quite interesting in my opinion. Mai Chau is another possibility - the stilt houses and a beautiful/peaceful little green valley.