I visited friends on secondment in London (July 2011) and decided a fun trip would be to take the train to Brussels for the weekend. We were not disappointed.
As far as the waffles go: I realize everyone recommends going to Dandoy, which I tried and enjoyed, but our favorites were from the small Belgaufra kiosks on the street. We ate four of these a day. You can add chocolate or other toppings to them but in my opinion it only takes away from them.
Chocolates: I picked up a 750g box of assorted chocolates for my girlfriend back in the states, and we absolutely loved them. They are a bit cheaper than Godivas, but most of the people we spoke to don't mind sacrificing a tiny bit of quality for that much more chocolate for your money.
A great place for lunch is called the Drug Opera. They have a wonderful daily soup special as well as great mussels. I recommend going with the ones cooked in a garlic cream sauce.
I feel the frites were overrated. To me they are just fries, and I've had better varieties in the states. Maison Antoine had an interesting take with different flavors of sauces to accompany them, but I don't believe my trip would have been incomplete without a visit.
Stop and eat at any of the little restaurants tucked away into the city's alleys near the Grand Place to enjoy today's catch from the sea.
Sit on the patio of a cafe inside the Grand Place to order dessert and coffee and enjoy the light and sound show in the evening on the hour (at least on Saturday). This is something we just stumbled upon but throughly enjoyed.