Hello! I would check on visit brussels to see if any festivals on, might be something fun to do. Eg at the moment there bruxelles les bains - beach stuff along the canal.
Am not sure I agree with Maison Antoine, depends where you are staying etc!
What are you into? Museums, going out, walking? There are some art nouveau walks, old Art Deco houses, the historical centre is great to walk around ( grand place is essential viewing, then around st Catherine, rue de Flandres) and the best bit is going from cafe to cafe and sampling the beers. I hear the royal palace is also open for free at the moment.
If you enjoy antiques / bric a brac there's the flea market on jeu de balles, if you are here on a Sunday, it's a nice Sunday morning activity and you can explore the weird and wonderful antiques stores on rue haute/ rue blaes.
On Thursday evenings there seems to be a few evening gourmet markets around...
Otherwise the atomium is famous and I quite like the castle museum by the Porte de Hal that shows all the city walls as they used to be...
Day trips to Bruges / Ghent also a lot if fun!
Anyways if you say what u like doing I can surely provide more tailored info, are you with kids?