One of the hardest things I have found while traveling with my limited knowledge of French and Spanish has been encountering unfamiliar idioms. They can be a big stumbling block to comprehension and conversation, even as vocabulary grows.
Here is a website with quite a few English Idioms.
It might be worth having the Project tackle a list of 25 or so each week, that the students can write in their journals or keep a handout, etc., and practice for periodic quizzes. (I suggest this because it would be yet another metric we can use to justify grants.)
HERE IS A SAMPLE of what the website has:
177 Idioms Beginning With ''