I pretty much love anything art nouveau. So whenever I go to European cities, I look to see if they have any art nouveau icons. In Prague there is Mucha, in Berlin there is the Bröhan museum, and in Brussels there is Victor Horta. There is a Horta Museum, as well as a number of buildings he and others designed nearby. A bunch of walking tours (like this one with a nice video) can help you cover the various buildings or get a peek from wherever you currently are via the pictures.
Like many architects of the period, he also designed the furniture, wallpaper, and interior structures like staircases. The museum has great examples of these. Alas no pictures were allowed and there aren’t any fair use pics. But if you’re curious you can google for yourself. Here are some pictures of his lovely buildings: