A grand park in the EU quarter, dominated by a massive triumphal arch that Leopold II commissioned to mark fifty years of Belgian independence. The arch itself can be climbed for rooftop views, and under its wings sit three substantial museums: the Art and History Museum (antiquities, decorative arts, and European archaeology), Autoworld (historic vehicles in an enormous iron-and-glass hall), and the Royal Museum of the Armed Forces. The park surrounds it all, making this a good half-day destination if you want a combination of museum time and outdoor space.
The Cinquantenaire is at the edge of the EU quarter, an easy metro or tram ride from the centre. Combine with a walk through the formal Leopold Park nearby and a look at the extraordinary Art Nouveau houses on Square Ambiorix just to the north.