It really depends on what you'd like to try doing; for example, if you'd like to try web development then being decent in javascript/css/html is important. If there's a field you want to check out, a library/framework aimed at that space might be worth familiarizing yourself with. Or, if you just want to learn something new without any purpose, then might as well try something as far from what you know already know as possible -- I guess in this case Haskell, OCaml, some Lisp, or even F# would be interesting areas.

EDIT: Or step outside the tech field into something unrelated, and see if there are any areas that might benefit from some tool you could create. Though I guess a month might be too little time for this.