Brussels is a very large city, with many neighborhoods to check out. I would advise you to see some neighborhoods other than the old center.
Ixelles for example, is a very lively and happening neighborhood with many cafes, restaurants and shops.
My personal tip is this Jazz-bar:
If you want to check out what the locals do in Brussels, there are many great tips on www.SpottedbyLocals.com/brussels
Bart about 10 years ago