Day trip to Brussels

I am after some advice from any well travelled readers of my blog. One week from now we will have a lot of time to kill in Brussels. Our flight home causes us to land there about 8am and leave about 5pm. We tried to change the flight but will loose our 46 kg luggage allowance down to 20 kg and I am telling you we can not do that! So the result is a forced day in Brussels. First we were none to chuffed about it. We have an overnight flight from Douala so will be tired, we will have heavy hand luggage to carry about but then we thought of beer and chocolate and decided not to fret. So dear readers any advice on what we can do with a day in Brussels to help us absorb and adapt to European life?