I'm currently living in Brussels. One of the great things about this city, is that it's quite small, and you can walk almost anywhere. The Grand Place is the main tourist area, and is fun to walk around, and look in the different shops. Most of the restaurants in the area are very touristy and nothing special, but Leon de Bruxelles (rue de Bouchers) is good for mussels.
If you like art museums, there is a good exhibition right now at the Museum of Fine Arts (Flemish art from the British Royal Collection). If you're interested in architecture, I recommend the Horta museum. The royal palace is open for the summer, and well worth a visit. It doesn't take very long, and is very impressive (and free!). There are lovely parks all over the city. The atomium (1958 World's Fair centerpiece) is also popular, but can get a bit crowded.
The best place for frites is Maison Antoine on the Place Jourdan (up by the European Quarter). You can expect queues, but it's well worth it. It's an outdoor stand, but you can take your order to the bars around the square, and enjoy the Belgian beers. We go there far more often than we should (just came from there, in fact).
If you get restless in Brussels, you can easily get to Bruges by train (approx. 1 hour). It's definitely worth a visit. One day return train tickets on the weekend are approx 13 euro.
A couple tips: Brussels is technically bi-lingual, but most people speak French. You will find that street signs are in both French/Dutch, which can be a bit confusing, as it can appear that the streets have 2 names.
