Seconded. Smaller Dutch cities are much nicer than the 'Randstad' (the cluster of large cities in the western Netherlands). Gezina's suggestions are quite good.
But to be honest, I don't think the Netherlands are the best Europe has to offer. I don't know what you consider the general vicinity of Brussels, but I can recommend the following:
Berlin. Beautiful and varied architecture, and much historical stuff (mostly focused on WWII and the Soviet era).
I'd also consider other French cities. Although Paris is nice, I prefer Lyon or Avignon. Both are beautiful cities.
Brussels. It's got chocolate.
Prague is also very nice, but it's a bit far from Brussels. Still, it's small enough to visit in one day.
Rome. It's probably too far from Brussels, but it's one of the most beautiful cities in the world.