When visiting any part of the world, one of the most interesting things to do is to find something that is particular to that place and bring it home with you.
Brussels, as well as the rest of Belgium is probably best known for beer. The problem is that you cannot always take beer home with you, or onto an aircraft. How about a beer stein? You can buy some that depict Brussels but might feel more German to you, as they do to me. There are also beer glasses of the best loved beers however, such as Duvel, Hoegaarden, Leffe, Delirium, Stella Artois and so on. They make good souvenirs.
For the durable keepsakes, you cannot go wrong with a mini Manneken Pis or Atomium!