Hi Ben (&sausage),
Vietnam = Friggin awesome :)
I went last year and discovered a new kite\windsurf\sup school + hire in Hoi An at Ang Bang Beach. So Sausage this is prob your best bet if you cant get to Mui Ne or Nha Trang. Mui Ne would be first pref though.
http://www.vietnam-kiteboarding.com/index.aspx?content=windsurf&lang=enwww.facebook.com/KbaHoiAnI think the conditions in Hoi An are more fluky but you can always go out on a sup or just enjoy the magnificent town that is Hoi An :) I would prob talk to the guys that run it.
Sausage - if you wingin it I would highly recommend Sapa (overnight train from Hanoi) it is stunning!!! Dalat is alos a cool place with lots of action, adventure like trekking and waterfalls etc. Not sure of weather that time of year, I went July but both Sapa and Dalat were a bit cooler which provided welcome relief from humidity.
Also Sausage, most Halong bay junks and itineries dont quite match up to what on offer and you prob find if you booked from here the price is tripple. We had to insist our guys took us to a place so we could jump off and swim, which they obliged but make sure you are clear what you are getting. Also very little shade on deck and no junk had its sails up :(
If either of you want more info on travelling in Vietnam, please feel free to contact me I covered Sapa\Hanoi\Halong Bay\Hoi An\Dalat\HCMC.
Jodi :)
SAPA - View from bed at hotel (Cat Cat View)

$30-40?
HOI AN