I'm just going to be missing you by a day (plan to be there on Friday the 12th). I've had great lunches at Cave du Roy http://www.caveduroy.com/ , located on the Grand Place (cellar level). You could make it as heavy or light as you wanted, including mussels (more than you can eat). I also had a great lunch several years ago at a little cafe next to it (looking at the Cave du Roy door, it's the first door to the left of it, also cellar level) and it had some lighter fare such as sandwiches -- unfortunately I don't recall the name of it. Both were very enjoyable and great atmosphere.