Try Gaelic for a really mind-bending language experience. I have a Swedish friend who lived in Danmark for a while and said it took her a month to start to understand the language(although the written form of Svenska og Dansk are really similar). I myself am used to the sound of Swedish (grandparents etc) so Danish sounds like they are swallowing their words. But that I can get used to. It is Gaelic that throws me....a phrase like "is maith liom" SOUNDS like "issmaylum"...fun times...!
Compared to German, I am having an easier time learning Danish...so far...at least as far as the articles are concerned, but the pronunciation for me is a nightmare. At least with French I became accustomed to masculine and feminine for inanimate objects (as ridiculous as that initially sounds), but with German there is masculine, feminine, and for added fun neuter. Then there is the whole range of articles to go with each depending on a wide range of scenarios and whether it is part of the subject or the predicate with no real indicator in the words themselves to hint that it may be either masculine or feminine or neuter (unlike with French). I initially thought French was challenging, and still have trouble understanding a native speaker if he/she is speaking rapidly, but now learning German and Danish, it seems like a breeze by comparison. As a native English speaker, I assumed learning some other Germanic languages would be easier, and I see now that I could not have been more wrong.