I got this idea from egadideas.com and used it in youth group last Wednesday.
Split youth up in teams and have them write a song or rap to the instrumental track. Give them at least 15 minutes, then have each team perfom in front of the audience. Award a prize for 1st, 2nd and 3rd places.
We had a blast as the groups took on different personalities and rapped about everything from Sunday school to chap stick (the chap stick rap is now posted on the wall in my office). 
For the rap music bed, I took the intro from an old Tonex song called Out of the Box and just looped it. If you’re interested in playing this with your group sometime, I’ve made the track available for download. I know it’s not a “real” hip-hop” beat, but it works. 
It will also be on my free resources page for future reference. Enjoy and have fun!
About me: I am married to my beautiful wife, 
February 10th, 2006 at 11:57 am
Very cool. I wish I could have seen that.
February 11th, 2006 at 10:47 am
I think the Chapstick rap was the best by far. The most original and unexpected. Funny game.