Why didn't I think of that?

Geek or not, if there’s one thing we all have plenty of it’s Cds. And if you want some exercise, but don’t happen to have any dumbbells around, just create your own with all of those CDs collecting dust. The one pictured is 10lbs. All you need are 150 CDs, a threaded rod and bolts and you’ve got a 10 pound weight.

http://www.slipperybrick.com/category/exercise-gadgets/