When my daughters were younger, I remember getting them a secret agent kit. One of the activities in the kit was to build a cipher wheel, which basically consisted of a smaller disk and larger disk fastened together. In my daughters’ kit, both disks had the alphabet printed on them, but they could also contain numbers or symbols. Once  a cipher key was selected, for example, the letter 'O' on the inner disk might correspond to the letter 'J' on the outer disk, my daughters then had a pattern set up that allowed them encode and decode secret messages.