Ask HN: Practical project ideas to use functional programming concepts?

I've been spending some time lately working on learning about functional programming and the various concepts surrounding it, specifically while using Clojure to simultaneously learn about Lisp.

What are your suggestions for some projects I could use functional programming (and preferably Clojure) and most of the concepts surrounding it for? I'd like that the projects would have enough merit to be interesting regardless of how they are made, but hit me with your best shot.