You've already mentioned drinking
But if you need to narrow down where you're going to do it, find the Delirium Taphouse. I can't remember which street it's buried in, but it's basically just off one of the main roads parallel to the Grand Place, and signposted once you get within spitting distance.

A huge selection of beer and a nice atmosphere. I met a Frenchman in there and promptly got him drunk.

Conversely, don't bother with the Brewing Museum under the town hall. It basically consists of two rooms, one of which shows a video about beer and the making thereof. Quite interesting, but, feeling that I hadn't got my E5's worth, I wandered round looking for more, and accidentally ended up in the offices of the town hall, looking rather lost..
(LongJohnBaldry, Wed 22 Apr 2009, 14:32, Reply)