A common 6-8 grade science fair project is a simple motor from a coil of mag wire, a simple frame (paper clips), and a battery & fixed magnet.

Simple components, easy to build, cheap, shows the principle.

Different variations on the theme:

google "make simple motor"

the usual one:

http://www.miniscience.com/projects/magnet_motor_kit/index.html

http://www.instructables.com/id/How-to-Make-a-Simple-Motor/

http://makeprojects.com/Project/Simple-Motor/67/1#.UE4e7CJX2W8

very simple hacks:

http://www.evilmadscientist.com/2006/how-to-make-the-simplest-electric-motor/