Bruge or Brussels
The poster that said to skip Brussels is being cruel. We just spent some days there with a day trip to Ghent. Ghent is definitely worth some time and has some nice cafes along the walking route, especially if the weather is decent.
Back to Brussels, if you're in the mood for seafood, the Ste. Catherine area has one street that has more than 10 restaurants that specialize in fish dishes. We also had a meal at Bonsoir Clara, mainly because it was close to our hotel. The food was 'Mediterranean,' and nicely cooked. A quail salad with figs was out of this world.
This report may be helpful, even though it is a few years old (things don't really change that much):
We enjoyed our stay in Brussels, and took day trips to Bruges and Antwerp by train. You could go to Ghent and other places by train as well. We stayed just up the street from the station, and very much in the center. Found some great restaurants. Brussels exceeded our expectations.
Belgium has more Michelin stars per capita than France. Even 10 years ago, so I find that a very harsh statement.
Brugge is lovely indeed but all that quaintness (and all the flocks of tourists) might get in the way of really appreciating all that Belgium could offer. Considering the size of the country I would try and visit more cities, or at least for dinner. I.e. Ghent is only a half hour train ride from Bruges and is similar (canals, medieval houses) but has a much more happening vibe and great restaurants like De Vitrine, J.E.F. and De Avonden (that is about to be run by the chef from Volta). Dulle Griet for an excellent beer selection or Jigger's for top artisanal cocktails in a speakeasy vibe. Brussels is perhaps less interesting for tourists but it has great museums (Musee des Beaux Arts) and the old city centre and market square, Sablon are nice places to spend an afternoon. But there is more big city ugliness, traffic, poverty, etc that comes with being the economic and administrative heart of Belgium (and by extend Europe).
Leuven, the biggest university town of Belgium is very close to Brussels as well and has a pleasant vibe for a night out though it gets too studenty during the week. Antwerp is only an hour from Brussels by train and has a sleuth of great restaurants, historic city centre with cosy jenever bars (De Vagant), a couple of great jazz cafés (De Muze is one of my faves) and an amazing cathedral. I can recommend De Veranda close to the Berchem train station for top food. Finally back to Bruges, I had a pleasant lunch at De Refter and for a splurge you can try the neighbouring big brother next door De Karmeliet. Also there are plenty of chocolate shops in Bruges, but for something truly special you need to go to the Chocolate Line (there is also an outlet in Antwerp).
In general Gault Millau and the Michelin guide (including the Bib Gourmand mentions) are not bad places to start if you are looking for good food in Belgium.