I would probably give the Belgian coast a miss at that time of year. I have been to Knokke-Heist in off-season (I think it was April) and it is ghastly - windy and rainy and dark (although Belgium is rainy at the best of times). The architecture is not so special there that you would have a lot to visit (although if you are a fan of Keith Haring the casino has some of his work).

I would just do Brussels with just 1 - 2 days up my sleeve, as there is quite a bit to see - although you would be wise to check open hours of museums across the Christmas - NY period. There is also a sound and light show for the winter season (the festival is called Winterpret / Winter Wonders) which lights up buildings in the centre at night. I am not sure when it ends (I think early in January) but you can google it to get an idea.

Lavandula