We did the into town trip with a lot less time than you.

The Musee de Beaux Arts is one of the great museums in the world.

The airport itself was fascinating. Because Brussels is the headquarters both of the European Union and of NATO, there is a lot of international traffic. Back in the day, there was a significant amount of traffic in minimally marked aircraft -- no airline names, aircraft numbers almost invisible. loading and unloading into unmarked trucks -- presumably black ops of some sort. Check it out.