Another simple craft for secret messages is whispering them into a homemade telephone! All you will need it 2 cups (start with paper cups!), some string, and maybe 2 paperclips or something else to keep the string from coming out of the cups.
Poke one small hole in the bottom of each of the cups. Parental help may be needed here. Thread the string through the bottom to the inside of the cup, and tie it to the paper clips so that the string will not pull back through the cups. Now have one person stand with one cup and position the other person on the other side of the room with the other cup. While one person whispers into the cup, have the other person listen to it by holding the cup up to their ear!
Why not try it with paper cups, plastic cups, even aluminum cans!? Is there a difference?