Marcolini has two locations on the Grand Sablon in Brussels. One is the flagship store which focuses on chocolate bars and confections, with a tasting room upstairs. Just a few meters away is a small patisserie/cafe where one can purchase many of the same chocolate items, but also enjoy sit down and enjoy a coffee and patisserie.
The Grand Sablon - not the Grand Place - is chocolate central in Brussels. In addition to Marcolini are Leonidas, Neuhaus, Godiva, Wittamer (the only true old-school Belgian chocolatier left on the Sablon), and Patrick Roger from Paris. Just off the Sablon are Pure and Alex & Alex and down the hill is Chocopolis.
If you are interested in chocolate the Sablon is the place to go. The Grand Place, except for the original Godiva store, is mostly tourist junk chocolate - cheap and cheerful where Guylian is at or near the top of the heap.