This is the birth place and home of one of the more unique breweries in Belgium (and that's saying something). Sure, you could visit the trappists til beer is coming out of every orifice (I'm intrigued...), but if you want something in Brussels proper or something that you won't be able to try anywhere else, Cantillon is a cool experience. The beer isn't for everyone-- it's certainly not traditional-- but the tour of the old brewery is pretty cool and I had a great time sampling the beers-- beers I've never had anything like before.
A local brewery the specializes in Lambic-style beers. Lambic beers are unique in that they use natural yeasts in the air to aid the fermentation process. I personally am not a huge fan of these beers because they are a little too sweet. Nevertheless, if you have never tried one it is a unique experience