Thanks for all the great advice and ideas.
I've actually been to Vietnam several times before including to HCMC and Hanoi. I actually love the hustle and bustle of the cities there. It'll be my first time in this area though.
What makes it a bit more difficult is that there are a few of us and backpacks are out of the question (not for me - but for some of the others!) which makes traveling around more difficult.
Looking at the distance from Da Nang to Hue (2.5-3 hours by train), we were thinking of splitting our time with three nights at Da Nang and three at Hoi An with a daytrip to Hue. We might need to reconsider as so many are against Da Nang.