I am writing in response to so many suggestions that any vistor to Brussels immediately head for the moules (mussels). It's not that I disagree, I've had some of the best here, but these are coming to be (particularly for the New York residents) a more available dish. Might I suggest some other local specialties, some that one will not find stateside?
try a dish with the crevettes grises (gray shrimp) found in the north sea and to my knowledge unavailable stateside. One can take them in a salade with tomatoes or in a croquette (fried with cheese).
Boudins here, white or black (blood) can both be quite lovely.
And finally, I personally seek out a filet americain upon each visit. This is raw beef, finely chopped to a creamy texture and spiced as well. (Yes, it is a steak tartare-- only aren't those served with egg and other things?) It is traditionally served with cornichons, picked onions and frites. It is a truly lovely dish and worth a try. for those wishing to try, but not go full force for dinner, try a toast cannibale.
The dishes are typically available at the bars that populate the downtown area (certainly for the one-day tourist, one can head to the Grand Place/Grote Markt to a place like the Chaloupe D'Or for a drink and a snack.
I've likely forgotten some dishes, but this is a start to the list of moules- alternatives.
This is not to say that one should not have mussels-- by all means do-- and without question the beers will be some of the best you've tasted, but there are other dishes worth a try.
Thanks for hearing me out in what might seem like a rant-- mild and loving surely, but possibly rantish.
Oh yes-- and to chocolates, there's really no going wrong although it is worth going to the Wittamer in the Sablon for some exceptional cakes.