I see Bruges mentioned, but not Ghent or Antwerp?!
Belgium, in my experience, ranks among top most attractive places to go if you're into architecture/historical buildings/medieval themes. All the cities are easy and fast to get to - very close to each other - and offer very different shots to be taken.
There's a harbour-cruise in Antwerp that takes about 4 hours and you get to see roughly half of it - one of the largest ports in the world.
Antwerp is the diamond capital of the world. Ghent is almost entirely cobblestoned and looks and feels like a medieval town, replete with castle, moat and all.
In Brussels - although it's a architecturally confusing place, has the Belgian Centre of Comic Strip Art and the building is an old Waucquez Warehouses in the Art Nouveau style. Absolutely amazing.
I lived there for almost two years without a decent camera and sometimes I wake up at night, many years after, screaming "NOOOOOO!" from the sheer stupidity of it..lol.
I guess the short version is: go to Belgium as part of your trip if art, culture and architecture has any interest.