In Brussels you should really drink geuze (tastes like a cross between really sharp scrumpy and champagne) and you can easily walk to the Cantillon brewery from central Brussels where you can get a tour and have a taste. It's not like any other brewery you're ever likely to visit!
Beers to try: Cantillon (Fou' Foune or Blabaer Lambik) and Brasserie de la Senne are two most local to Brussels. But any geuze, such as 3 Fonteinen, De Cam, De Troch or Boon is not going to have come very far. The superstars of Belgian brewing are probably De Struise and anything by them is worth trying (esp. Pannepot and Cuvee Delphine). It's all very well trying the Trappists, but they haven't got much to do with Brussels.
Bars that have good beer selections:
Le Poechenellekelder
Chez Moeder Lambic (there are two)
Restaurants with good beer selections:
Nuetnigenough
Les Brigittines
If you're there on Sunday, a trip out to Eizeringen (about 10 km to the W of Brussels) to visit "In de Verzekering tegen de Grote Dorst" would be worthwhile, but check on the exact opening times first.