What language would you recommend?

I would like to learn a language, but I am undecided which one to choose. I am Spanish, so languages like French, Portuguese, Italian, etc. don't interest me because they seem too easy and that puts me off. I also know enough English. What I am looking for is a language with a different structure than the classic one, but without being as impossible as Mandarin or Arabic. I don't learn it for work opportunities or anything like that. Just for fun. Which one do you recommend?