You can turn the disks individually (or not). You can lock screen orientation and turn on vibrations.
You can cipher and decipher a message and display the frequency analysis for each disk.
You can break a message searching some keywords.
Note for teachers : in order that your students familiarize themselves with the disk and, initially, only use it (on a tablet available at their disposal, for example), you can first disable the other activities by setting a password from the main activity. Then, you can enable the other activities later.