Maison de la Science.
As a little boy, too many moons ago, I visited this museum with my parents. I remember that I stood in awe for the many skeletons and stuffed animals, so I had to visit it again. The bad news is that there hasn't changed a thing, including the (mainly French) tags. The problem is not so much the language (you are in the French speaking part of Belgium after all), but that the tag for a stuffed pigeon will say just that (together with the Latin name.) For a museum that goes by the name House of Science I find that hardly educating. However, for people who into that kind of stuf (pun intended) there are lots of stuffed animals and (if you digested that) at least as many skeletons (also from a mammoth that was found nearby and from a fin whale.)
There is some good news however: the entrance is the price of a coffee, the building which houses the museum is beautiful, there is is an aquarium in the basement and if you don't fancy all that, there a pretty fresco of the famous Belgian painter Paul Delvaux on the ground floor.