Assuming it is your first visit to Brussels, and if you have the energy, it would be a pity to remain at the airport with so many things to do nearby.
There is a train from the airport to Central Station which takes 16 minutes and runs 4 times per hour. With a half hour journey each way, and one hour of buffer time, this leaves 3 hours in Brussels.
If the weather is good - always a factor in Belgium - you can do the classics. Within half a km from the Central Station, you have the Grand Place, Royal Galleries, The Sablon Church & Square, The Royal Palace, the Cathedral, and so on. Lots of photo opportunities.
Or you can take one of the city walks. There is a walk around Brussels' 40 comic strip walls, which takes you all over the town. You will not be able to complete it in 3 hours, but at least try a good portion. There is also a walk around the European Institutions and there are several walks organised around Brussels' few 100 Art Nouveau buildings.
If time allows, take a break and have a waffle at Cafe du Vaudeville in de Royal Galleries, or a beer in the Roy d'Espagne on Grand Place.
If the weather is not good (not a small probability), go to Mont des Arts and visit for example the Magritte museum. Combine it later on with above waffle or beer.
