Forget sand or water and plastic bottles. Just take a PVC tube, stick a ball bearing inside and cap each end. Instant cheap scoring object that everyone can make as many of as they want. No mess and dynamically changing COG. Make some short, make some longer for different point values, etc. No more scavenger hunts at WalMarts for the illusive orbit balls, etc. either. Kind of like this:
http://www.chiefdelphi.com/media/photos/22847