The Cantillon Brewery is one of the few that still makes the unique gueze and lambic beers. The brewery was started by a family and four generations later, that same family still runs the place and brews the beer.
You can tour the place and even drink beer there. For the price of 8 euro you can get a tour and sample two different beers.
The brewery is a short distance from the south train station. Best to take the train if you're going to drink!