In case you are able to travel the 11th of November you can attend the yearly commemoration of the WW1 Armistice in Ypres. The military ceremony with the Last Post will be under the Menin Gate around 11h AM, but being earlier there will give you a better place to see it.
As already said Leuven is nice and Mechelen too, both an easy ride from Brussels with the train. Lier is lovely with it’s “famous” Zimmer tower and museum about clocks and astronomy. Also easy to get with a change in Leuven or Antwerp-Berchem. https://zimmertoren.be/?lang=en
Aachen with it’s Dom and Domschatzkammer (both on the UNESCO world heritage list) is certainly interesting from a cultural and historical point of view. Easy to reach with highspeed train from Brussels.
Lille just across the French border is a true shopping paradise and offers enough indoor activities if the weather won’t cooperate, not unusual this month. It has a good museum of fine arts and the birthplace of Charles de Gaulle is a museum too. Easy to get with high speed train and worth considering to my opinion. Back in Belgium nearby Tournai or Doornik in Flemish is a real of the beaten path destination, not big but famous for it’s cathedral and the little fine art museum is not outstanding but nevertheless nice. Likely you will be the only English speaking tourist there. Easy to reach from Lille.
I agree with Waterloo with it's modern informative museum and from the top of the Lion's Mound you can oversee the former battle field.