With that many instruments, you may need to jazz it up a bit. Pick a metal piece that you like, play the main riff and the chorus, and then spiral off into riffing variations and solos. Or, you could take a non-metal instrumental and play it with a metal twist. Personally, I have always wanted to hear a black metal band of some kind cover "Careful With That Axe, Eugene" by Pink Floyd. It's very sinister and edgy, with keyboard solos and some primal screams to boot. It even has scope to slot in some crashing guitars, and maybe even a feedback-laden solo.
But it's up to you, mate... don't do anything that you and the band don't feel comfortable with.