I'm your man for Vietnam - my favorite destination in SE Asia. Chiang Mai is just too civilized for me - I far prefer the more developing countries, and you won't find a more interesting place than Vietnam. No, not as slick or pretty (Chiang Mai is half gold with all those Wats!), but Vietnam has more charm and my pick for the friendliest nation.

You could fly into Hanoi and return to HK from Danang - Hanoi for the wonderful culture and shopping, and Hoi An (Danang Airport) for relaxation and great food. Buy your VOA visa prior to travel and if you need any help, I know an excellent agent in Hanoi.