Not used by the royal family anymore, but still worth mentioning : the royal villa in Spa.
This charming villa with beautiful horse stables was built in 1885 and bought in 1894 by queen Marie Henriette (wife of king Leopold II). She lived there untill her death in 1902.
The villa is now used by the city of Spa, mainly as an exhibition space. I believe it officially still belongs to the royal family.