You could demonstrate how the readings vary too much with the .5 interval. Try to steer them toward 2 or 5 minutes, depending upon your lesson plans. Students could select up to three materials, use a one-minute interval and vary the distance from the Geiger counter to see if distance affects the amount of radiation. Discuss nuclear power plants and safety procedures for shielding and handling of radioactive materials. Also, discuss possible environmental hazards, such as polluted water, air, soil.
Health risks are also interesting to students, particularly regarding mutations and genetics.