Jabberwocky too, use a language unknown to man, tell them to work out a piece of drama based on a certain theme in groups, then later on, tell them to perform it.
After they've performed it, don't let them go back and rehearse it, just let them improvise, using random wording, for instance take out all the vowels or the consanents, and then use spontanious improvisation to try to create the same scene in this new found language.
You should be able to work out what they're trying to say by their movement, the facial expressions, the way they say it, etc, it's quite challenging, but when you get going, it's really fun too