When two Gencons are connected to each other, one can act as a generator and the other as a motor.  Students can feel the load of a back emf when they each try to turn the Gencons while connected.  Don't let them crank too hard, they might break.

They can be used to charge a large capacitance, such as the 1.0 farad capacitor shown.  The capacitor will discharge through the Gencon once you stop cranking, and the Gencon will work as a motor.  See if your students can explain why the handle continues to turn in the same direction you cranked it while the capacitor discharges.