Heltec V4 or later If you have more time than money, try the Heltec V4 or later, currently one of the cheapest boards available at around €20. It has a postage stamp-sized OLED screen, a couple of tiny buttons, WiFi/Bluetooth, and USB-C input/power (but use a USB-A to USB-C cable). Received messages are displayed on the OLED and can be cycled through with tiny buttons. Sending messages requires connecting to it via WiFi or Bluetooth. It has no case, but the little plastic box it comes in can easily be turned into one with a sharp pen knife. It also has no battery, but it is a good idea to have a separate power bank anyway since you need a working phone or computer to send messages. It has no GPS.