During my summer vacation in Brussels I visited the four inscribed Horta houses. The Horta Museum is lovely and nicely overwhelming. An absolut must. Since the other three houses are not open I booked a tour on Arau.org. They make an Art Nouveau tour every saturday and visit two houses (not necessarily by Horta). In our tour we visited the Maison Autriche and the Maison Eetvelde, both by Horta. The first one is a museum that you can visit independently but the second is a privat house with a very nice interior. In general though I would not really recommend this tour. The group was very big and the tour was given in French and English by one guide. She was decently knowledgable but she often made mistakes and she gave all the explanation in both languages. Sometimes the group was split and entered the house in turns. All of this meant a lot of waiting, lots of repeats. And in the end it was a three hour tour and only ten minutes in the Maison Eetvelde, that you couldn't enter normally. If you know a little about Art Nouveau the only reason to take the tour is to get inside one of the private houses but be sure to check beforehand which houses are on the schedule.

Otherwise I recommend to visit the Horta Museum and the other Horta houses that are open to the public like the Maison Autriche and the comic museum. The Maison Tassel and Solvay are also impressive from the outside. But as important Horta is there are a great many other brilliant Art Nouveau buildings by other architects in Bruxelles and in some areas like Ixelles, Saint-Gilles or Schaerbeek you can wander for hours and discover one gem after the other. Some of them are covered by the nice map Bruxelles Art Nouveau by ARAU which is available at many Art Nouveau museums in several languages. Some very nice and open Art Nouveau houses are the Hotel Hannon (Espace photographique Contretype), the Old England (Museum of Musical Instruments) and (as far as I have heard) the Maison Cauchie. In addition there are very nice displays of Art Nouveau furniture and art in the Musée des beaux-arts and the Musée d'art et d'histoire. I have certainly never visited a city with so much Art Nouveau. There is a lot to explore even beyond Horta and a much of it is quite wonderful.