I visited Brussels for a week in April and didn't come close to exhausting the possibilities -- I'd need another week just to see the picture galleries and take some side trips! It's a medium-sized cosmopolitan city, easily explored by public transportation. I liked that it hasn't all been turned into a theme park -- it's a city in the process of reinventing itself.
If you like architecture you'll have plenty to look at -- in addition to the (rebuilt) medieval splendor of the Grand' Place Brussels has superb Art Nouveau buildings, and they're all over the place; I went to some very interesting small museums about 15-20 minutes from the center of town. I attended first-rate classical and new music events almost every night. The European Parliament presence means high hotel rates for the larger high-end properties, but I got decent deals on Priceline (though if I were to splurge I'd look into the Dix-
Septieme and the Dominican)
The Grand' Place and the Atomium were absolutely not the highlights of my trip -- I can understand visitors' boredom if they think that's all there is in Brussels. (though I went to the World's Fair 50th anniversary fireworks at the Atomium, that was fun)