Spent 5 days on the Belgian coast with the family a few weeks ago at a Centreparcs look-alike.
Whilst it was pleasant enough I would recommend making a special journey there.
There’s nothing to see particularly. The coast is wide, flat sand with dunes behind as far as the eye can see – you can see how they managed to rescue the BEF from just down the coast in 1940. Ostend has a wide promenade and some boat stuff in the harbour but only a few hours worth. Brugge was definitely the sight-seeing highlight.
On the other hand my Dad went to the Ardenne region recently and loved it. Rolling forested countryside and lots of war-related sites.