Meshtastic is "An open source, off-grid, decentralized, mesh network built to run on affordable, low-power devices". The network supports text-based messaging and also sensor date from things such as temperature and humidity sensors, and GPS trackers/receivers. The frequency band used by Meshtastic is licence-free so there's no significant barrier to entry.
Meshtastic hardware comes in the form of low-cost kits, generally based on the ESP32 or the low-power Nordic nRF52850 microcontroller. With the right kit, you can make desktop, handheld or outdoor, solar-powered Meshtastic nodes. You can buy a case, or 3D print your own.
Having made up your radio/node you can start chatting to family, friends and like-minded technical people, both locally and often at some distance, using the open channel or your own private one. Here's the current state of the Meshtastic Mesh in my region:
As well as the interest of regional messaging without the use of a telephone network or the Internet, building and setting up Meshtastic devices is an ideal STEM group activity.
Here's one I was making earlier:
Resources: