That should be a great trip! I hope you have the opportunity to visit other areas as well. I will forever think of Ho Chi Minh as "Saigon" and visited there very briefly in September of 2012. I found it very industrial and not so appealing while Hanoi is a lovely city having had a heavy French influence. Halong Bay in the north was a once in a lifetime bucket list experience for me! I hope you can see that some day!
Da Nang is near Ho Chi Minh; I have not been there but my husband traveled often for work and the old city of Da Nang is the craft, silk, embroidery, wood working, etc., center for Vietnam. Marble Mountain is near the airport on the way into Da Nang and we have these big marble lions that hubby had shipped over from there.
If you are like me, you will be amazed when you see all the motor scooters in the streets! And you'll be snapping photos of number of people and items they pile onto them! The Vietnamese local who worked with my husband, and was my guide, told me when crossing the street to step out with confidence and do not hesitate, just "GO!" He said not to look at them. It was really scary as they buzzed about me! I was so glad to have him with me. Oh, and some shops expect you to remove your shoes before entering.