For those interested in the life in the sea a visit to an aquarium can be great. This is especially nice for families with small children and it can be a nice activity on a rain day (see: climate). To get to the aquarium travel with the metro to Simonis, and continue on with tram 19 from there to the stop Bossaert-Basilique. The aquarium is not far away from the fifth largest church in the world, the Basilica of the Sacred Heart.
The aquarium is normally open from 10.00 till 18.00, but it is closed on national holidays. The entrance fee is 8 Euro for adults and 5 Euro for children. It is not the largest aquarium you have visited (probably), but if you are looking for a nice indoor activity some day with children, this is a nice Brussels activity.
Av. E. Bossaert, 27 1081 Brussels