"Was für ein schöner Garten."
Why is "ein schöner Garten" not in the accusative case ("einen schönen Garten"). Is it not the object of "für"?
I believe 'für' here isn't acting in the normal way but is rather an idiomatic emphasis thing. If someone could explain that a little better that would be great, but I think that is the gist of it.
Rather not. You might hear that in a very colloquial way in some dialects and areas of germany, but that's nothing you should learn. What a beautiful day = Was für ein schöner Tag. What a mess = Was für ein Durcheinander. Was für is an idiom in german you should get used to. Was für can also be 'What kind of', http://duolingo.com/#/comment/163012