Hi, I think that adjectives are very important in any language and can be quite tricky. So I find the exercises for adjectives in trying to learn Danish extremely short and limited and I would welcome a more extensive range of phrases and vocabulary.
I see that you have gone quite while without an answer so I will attempt something here. I must admit that I do not have a great answer but I do have a bit of an answer. For some of the complexities of the adjective, beyond what Duolingo offers, I suggest Basby.dk.http://www.basby.dk/modul1/. Memories has at least one specific course on Danish adjectives. http://www.memrise.com/course/53407/danish-adjectives-by-theme/.
Hope this helps. And best wishes with your study of Danish!