Walk, jog, or run across America. Maps of individual states, visual progress, competition, and rewards, seem to be great motivational ideas. Make sure that the distances that each class has to walk or run are the same, because getting across Rhode Island takes a lot less that getting across Texas. Also, try doing this on a 1/4 mile track as it will help keep your youngsters honest and they will be able to know just how far they have run each time.