I recall having great fun with using the(very much unknown to most people at my workplace) NET SEND command from a DOS window circa mid 90's to late 90's to send messages to particular staff.

In those days there were many (especially older) staff who had never ever encountered the little popup box with a message before.

One such instance with a very novice computer user I created an entire regime of exercises to combat OOS / RSI.

Every so offen, when I remembered, I would send this person a message telling them it was time for a compulsory exercise break. Sometimes I had them stand up and stretch, sometimes do arm exercises, sometimes wrist ones etc.

This carried on for probably far too long actually... but it was great fun and the person took it in great humour when told what and who was behind those messages...