Haskell is surprisingly fun to hack about in. You've got a fast dev environment, a huge selection of libraries to play with, and a syntax that can pretty much be manipulated into any style of programming you want. It's also one of the few languages in which you can tell your code works by how sexy and sleek it is.

It doesn't get as much press for tweakability as Ruby, but you can definitely muddle and muck around in Haskell for the sheer sake of creating something.

Check out #haskell on Freenode. Between the category-theoretical arguments, you'll find plenty of hacking for the sake of hacking.

edit: check out http://www.willamette.edu/~fruehr/haskell/evolution.html and tell me you don't think such a flexible language requires a sense of play.