We spent many weekends there when we lived in Holland – great place, especially outside main tourist season.
Avoid the obvious tourist traps around the main square for food (OK for beer though!) and just have a wander – it's a beautiful city at night and some of the best places are just neighbourhood bars in the back streets. If it's busy it's probably good.
Couple of recommendations, although it's been a good few years since I was last there:
This place on Wiijngaardplein (near the Begijnhof) is a fantastic old building and certainly used to do great steaks and seafood:
http://www.maximiliaanvanoostenrijk.be
Cambrinus has a fantastic beer selection:
http://www.cambrinus.eu
The bar at the Straffe Hendrik Brewery is worth a visit too:
http://www.halvemaan.be
Other than that, if you're there early enough and it's open, would recommend a visit to the Basilica of the Holy Blood on Brug square for a genuinely spooky experience! Evening boat trips are also good fun, but doubt they're running this time of year.
Have fun!