Okay, you will see probably most tourists coming here, BUT it is really worth it and beautiful. You are in the area that will send you back into the past and probably one of the most preserved in Brussels.
At night the Grand-Place has a beautiful illumination and you will see spending 20 minutes trying to photograph every building there is. Make sure you look at the roofs and try to tell what the statues on top of the buildings represent!
Normally you are not allowed to stay sitting on the floor (even though you see drunk people doing it!)