I just came across, during a further exploration of the Pearson ELT Community site, their idioms discussions space.
There doesn’t seem to be much in the way of discussion, but they have posted a set of mini-videos which purport to explain English idioms and expressions. The videos are very short (about a minute) and are followed with a dictionary definition. One of the tasks they give is “Can you guess the idiom before the definition comes up?” If you had learners in teams with different coloured board pens, and they raced to write the expression on the board before it came up, it could work….
The videos are also available via the Pearson You Tube channel (I’ve tried to embed one of them below, but don’t think it’s worked – so click the link instead).
Here’s the original page again: ELTCommunity.com: Space: Idioms Discussions.