Royal Museum of the Armed Forces and Military History (Brussels)
This is a large museum that catalogs Belgium miliary history and equipment from the 18th century on inside a park (Parc du Centquantenaire) in Brussels. 
When I say large, I mean large, its packed with display items. Whether you want to see 18th century uniforms, or ww1 mortars, or aircraft its here. Not just belgium aircraft, but also great examples of Russian hardware. A mil-24 Hind, yup, there’s one. Definitely first time I’ve encountered this helicopter displayed anywhere. Russian royal jewelry ? check. Paintings, swords, banners, Mirage III, f-16 ? check, check, and check. I was here for about 4 hours and ran out of time. You easily do a day here if you like military history.
link to their website – klm-mra.be/en