I would like to recommend an Art Nouveau bus tour operated by ARAU (Atelier de Recherche et d'Action Urbaines), a non-profit local resident's group of architects, designers and interested citizens. The tour takes you to the most extravangant houses of the time around 1900, explaining not only who built it, but also who lived in it and what happened to the building throughout the century. The tour includes visits to the interiors of some Art Nouveau buildings, some of which are not open to the public.
The tour guide we were lucky to join was a very entertaining man, who also told us a lot about city developement and the way Brussels deals with its historical monuments.
The tour takes 3 hours, price is 17 Euro, online reservation possible. For details see: www.arau.org
Google map: tinyurl.com/nmfoha