In reply to mypyrex:
Plenty of decent restaurants and bars around the Place de Luxembourg though none stand out from memory. Usually full of Eurocrats letting their hair down after work but that shouldn't be an issue at the weekend.
One place I've been to a couple of times near there is Poivre et Sel on Rue du Parnasse.
http://www.poivre-et-sel.be/fr/ Cosy, informal and reasonable food. I was on expenses but it didn't seem too expensive as city restaurants go, certainly better than most around the Grand Place.