This Activity is meant for Boy Scouts.
Decide for yourself if it is appropriate for your younger scouts or not.

paper and pencil for each Writer.
signal flags (or flashlight at night) for each Sender.
Flag for leader.
Written message for each team.

A simple message that uses many different letters and can not be easily deduced is good - like 'See the brown fox'

This can use signal flags during day or morse code with flashlights at night.

Each team consists of a Runner, Reader, Sender and a Receiver, Writer, Runner. So, each team needs at least 6 scouts. If you have less, you can combine Reader/Sender and Receiver/Writer. If you have more, you can add more sending and receiving stations.

The leader stands at a defined location. The Runner/Reader/Sender (RRS) for each team is positioned at least 150 yards away in different directions.
The Receiver/Writer/Runner (RWR) group for each team is positioned at least another 200 yards away from their RRS group, preferably out of sight of the leader but able to see the RRS group.
The leader has an identical written message for each team.

When the leader waves his flag, the Runner from each RRS group runs to him, retrieves his message and runs it back to his RRS group.
The Reader reads the message to the Sender who signals the message to the Receiver in the RWR group.
The Receiver tells the Writer the message who writes it down and gives it to the Runner.
The RWR Runner sprints the message back to the leader.

Points can be awarded for quickness and correctness. Or, if it is incorrect, the runner can be sent back to request the message transmission again.