Write a game. ASCII Tetris or Pac Man or Space Invaders would be fine. Whether that is a simple afternoon activity or a somewhat epic undertaking might tell you something about the language.
I've been writing a couple of small games with Clojure + Swing as a way of learning Clojure, and it's been very smooth so far. I haven't even touched concurrency or macros yet but the basic language feels even more consistent and logical than Ruby.
