Loads of AB contributors have labelled Brussels as the most boring capital city in Europe. I tend to agree, but that doesn't mean that it's not got certain charms.
The most photographed sites are the (unmissable) Grand Place and, of course, the Manekin Pis. (I highly recommend the Taverne Manekin Pis, which is on the street corner diagonally opposite the famous statue. Good Belgian beers, a pleasant atmosphere and superb - but quite cheap - food. Try the rabbit casserole!).
To see what else Brussels has to offer, start here:
http://www.brusselsinternational.be/wabxlint/e n/visitor/brussels-tourism-visit.act
However, for me, the best thing that Brussels has got is the range of frequent (and fairly cheap) train services to more interesting places in Belgium. There are trains to the beautiful city of Brugge every half hour. (The journey from Brussels Zuid/Midi takes under an hour. Trains usually leave from platform 15)
http://www.brugge.be/internet/en/index.htm
Ghent is part way along that line, and also worth a visit:
http://www4.gent.be/gent/english/tourism/index .htm
About 15 minutes beyond Bruges, you arrive at the vibrant coastal resort of Ostend, which I love. (There are occasional through trains but you normally change at Brugge. The total journey time from Brussels is about 1� hours).
http://www.toerisme-oostende.be/index.phtml?la nguageid=3
Chris