Most museums are closed on 25/12 and 1/1, but not all of them! I’m pretty sure you’ll find something that will tickle your fancy. Museums open on legal holidays? The best invention since sliced bread.
For those who like cars: http://autoworld.be/home
For those who like old toys: http://www.museedujouet.eu/index.php/jouet/gestpages/pageenglish
For those who like chocolate: http://choco-story-brussels.be/en/Homepage-en
An art collection displayed in a beautiful Art Deco house – and it has a nice garden as well (my personal favourite in this list): http://www.museumvanbuuren.be/en/home.php
And for those of you who are still here on the 2nd of January: https://www.comicscenter.net/en/news-flash/opening-hours-during-christmas-break ==> famous Belgian comics in a wonderful Art Nouveau site by Victor Horta.
Enjoy you first day of the new year!