Hi Juan,
For me, the places to visit in Brussels are the "Grand-Place" (one of the most beautiful in Europe), the area of "les Marolles" with the "Place du jeu de Balles", very typical and still with the traditionnal style, the "Mont des arts" and the Museum of music (old england style). You can also visit the houses of Victor Horta, they are famous. Two days is a short time :-). On the evening, you can go to "La bécasse" where you'll be able to try the Lambic bier or to "Delirium" to taste more bier. Or, just in the front of "Manneken Pis", one of the oldest bar of Bruxelles. More strange, near the Grand-Place, you can go to "Au goupil" where you'll be able to taste wine of fruits. At least, I you need and if I'm free, I can show you some places. Have you already place to stay?