Brussels has many Art Nouveau buildings, and the person who designed and inspired others was Victor Horta, whose home has been turned into a museum.

The balcony of Victor Horta’s Art Nouveau House Museum in Brussels, Belgium.
No photos were allowed within the house so you will either have to go to Brussels and see it yourself, or be content with exterior details such as this door handle.
This booklet on the Musee Victor Horta showing some of the Art Nouveau details of his home-turned-museum in Brussels, Belgium.Victor Horta designed many places around Brussels. Another that is easy to get into (for a fee) is the Comic Book Museum where, along with admiring all the Art Nouveau detailing, you learn all about cartooning.

Around March of eatery year a group called BANAD offers guided walks and tours of various Art Nouveau/Art Deco buildings in Brussels: http://www.banad.brussels/en/tours-events/guided-walks.

More on Belgium (and Brussels).