The Navy signal flags were brightly colored to make them easier to read during the smokey, hazy conditions of battle. The Navy had a flag for each letter and would hoist the flags up the mast. Divide the class into groups. Have the groups write a message and send it to the class members. The remaining teams try to decipher the message.
Younger students can just spell out their messages older students can create a cipher.
Students may wish to make flags out of cloth and send messages across a field.