The Atomium has a magnetic attraction. It is a sculpture? A building? A wtf? I don’t know exactly how to define it, but it seems cool to me.
When you are in Brussels, you feel that you have the urge to go there, although you know it will be a bit expensive: Yes, you have to pay if you want to visit the inside! There are exhibitions, a restaurant and the possibility to make good photos from the top, if the day is sunny enough.
You can avoid paying the entrance: Its no a great deal, but you should make some photos on the outside. Maybe the best its doing that. And and night the illumination, with a handful of leds on the spheres, its amazing.
You can arrive there taking the metro (line 6, Heysel stop)
Final tip: The stairs can be claustrophobic, I don’t recommend to go inside to those people suffering in closed spaces.