Originally Posted by ravenheart
Rodrigo Y Gabriella's covers
This, definitely. That's the first thing that came to my mind.
Also, depending on whether they want these to be "semi-danceable" tunes or not, you could consider the String Tributes to Tool album, and also the Apocalyptica covers of Metallica and a few other metal songs they've done.
I'm sure there are more things to consider. If I think of any more I'll let you know.
EDIT: Also, if "Rusty Cage" by Johnny Cash qualifies, why not "Hurt" as well? That's one of his best covers ever.
Originally Posted by brutal_descent
Richard Cheese. A lot of Richard Cheese
This could also be a really good idea, as long as the crowd in general has a good sense of humor.