Ixelles in the south is a nice area, quite smart/bohemian but with nice parks and lakes. The grande place is awesome but of course super touristy. If you like Beer (who doesn't ?) there's a bar called Delerium just near it which has 2000 different types!
Food is great, definitely try some moules frites and the meat, especially the lamb, is probably the best I've had anywhere. For food probably best to avoid the super touristy food street off the grande place. The food is okay but the atmosphere is crap. Fin de Siecle in Ste Catherine is a good place to eat, huge tasty portion, very popular with bruxellois and it has a good atmosphere. There's also meant to be a really good stand up seafood place in Ste Catherine for fresh lunchtime mussels and oysters.
You've got to try the frites as well. The best two friteries are in place flagey in a little shack place by the lake. Long queues but worth it the other one's called something like Antoinette in Place Jourdan is meant to be the shizzle as well.
Place Sablon which is maybe 10 minutes walk northeast of the grande place has some nice bars and is a good place for a relaxed drink. Some nice looking restaurants as well.
For culture there's loads of stuff to do. At the moment they have lots of free outdoor music things happening so worth looking into them. I'd recomend the Magritte Museum as well but its just opened so the queues might be pretty ridiculous.
Phew, that's about it. I've just left having spent 5 weeks there. Its a cool place. Watch out for the weather though, definitely take a good umbrella.