Belgium -A much under-rated country. I've taken my bike a couple of times and loved it. The Ardennes (in the South and East of the country) are fantastic for cycling, and include some pretty challenging hills (note the altitude in the "High Fens" shot) and an opportunity for border hopping into France, Luxembourg and Germany.
Beautiful countryside everywhere in the Ardennes. These shots were around Bouillon:
There are many lovely river valleys to ride along. These were taken on the River Meuse cycleway at Yvoir near Dinant, and in La Roche en Ardenne:
The North is nice and flat, if that is what you are looking for, but a bit dull to my mind, though my wife and I just love Bruges and, (pictured below), Ypres:
From what I have seen, the cycle paths are much better in Holland, but I'd choose the hilly parts of Belgium anyday. Cheers,
Donger.