Here's a simple design for a mini copter that really works! After you build it, you can experiment with changing the design; change to heavier paper, add weight, or fold it differently, to see if you can make improvements.
Follow the link to this page, where you can see a full size design that is ready to be printed out on regular paper and constructed. The page includes instructions.
Be sure to drop it from high near the ceiling ... it should work perfectly the first time. We've seen a cardboard version of this design successfully used in an egg-drop contest to safely drop an egg from school roof height to the ground!