We went specifically to see art nouveau architecture, but it proved rather elusive until we bought a 'Five Art Nouveau tours around Bruxelles' walking map at the Horta museum. This made everything accessible, and gave excellent summaries of art-nouveau style, museums, and brief notes on hundreds of buildings.
Visit Brussels on the first weekend in the month, when the wonderful Cauchie house is open, but be prepared for an enthusiastic and lengthy address in French!
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