I have been to Hanoi and HCM but would be interested in other places that are good for a bit of touring and a bit of relaxing (with a 4 yr old!) Which are best Danang, Hoi-An, Nha Trang, would be Sept/October time- can you combine any of them?
Hoi An is 30 minutes from Da Nang by taxi, Hue is probably 1 or 1 1/2 hours to the north - a nice leisurely train ride also. Nha Trang is quite a bit farther south and you would probably need to fly to Cam Ranh and drive 45 min to Nha Trang - it is an overnight (or all day) train ride. Hoi An and Nha Trang are popular tourist places, I am not sure about Da Nang, although it does have a beach.
Hue, Da Nang, Hoi An would seem to be the most logical combination to me.