There are two reasons why you should visit the Royal Museum of the Armed Forces and Military History during your stay in Brussels: 1) it is PACKED with history. This is in a literal sense, it is difficult to see the walls in the enormous halls with every piece of military history you can imagine displayed, explained, and shown. 2) The hidden panoramic view of the city.
I’m a bit of a museum geek, but I understand that others might find looking at historical explanations to not be the most exciting thing. The view of the city and the Cinquantenaire Park is amazing though and almost worth the ticket price alone! Make sure to do this well before the closing time of the museum because they usually close the doors to the small staircase 1 hour before. Once you get upstairs you will find a relatively small rooftop with a picnic table for you to take the view in.
It is also a very kid-friendly museum, not to read all the descriptions of the various wars Europeans fought all over the world, but to walk through the aviation hall with real-life airplanes standing and hanging above your head.