Whats an interesting language to learn and why?

I've dabbled in spanish, german, russian, japanese and chinese, and am deciding which one of these I would like to take to a proffesional level. But what I would like is another language to dabble in on the side as a hobby when i am burnt out on studying the ones I have taken to proffesional level. I dont care about number of speakers etc. but an quick overview of grammar etc. would be helpful if you suggest a language. So far i've been considering icelandic or gaelic . . .