Here are a couple must-see spots, if art nouveau (Jugendstil in German) is your thing (I love it, also!).
First, go to the large cafe at Namesti Republiky, on the edge of the Old Town. It is classic in style and atmosphere.
http://www.timeout.com/prague/restaurants/venue/1%3A17519/kavarna-obecni-drm
Next, go to the main train station and go to the "old part". Look up and see something worth gawking at! This ceiling/dome is famous, but most people never take time to see/visit it.
Then just wander around Prague and take in the numerous buildings from the heydays of the Austro-Hungarian Empire, of which Prague was an art center.
http://www.czechtourism.com/a/art-nouveau/
https://www.google.com/search?q=jugendstil+prag+geb%C3%A4ude&espv=2&tbm=isch&tbo=u&source=univ&sa=X&ei=a6LeU8m8DNKryATD8YHQCA&ved=0CBwQsAQ&biw=1280&bih=689
Finally, I hope you are planning to visit Vienna, which will give you even more "Jugendstil" (Klimt, for instance).