I used Jelly Beans in my yr 10 class instead of beans as students enjoy eating them afterwards. Obviously that might not be suitable for every class. The additional benefit was that I could ask how they could encode more than one number with the Jelly beans. The answer I received was “to know the initial number of jelly beans for each colour and to encode one number (message) per colour”.
Next time I teach that lesson I will try to hand out the materials (beans and cups) first and ask students how could it be used to encrypt. They might not be able to respond with the correct concept but it might get them thinking.