Even though sending over Hello, world! is already pretty cool, it is not very useful yet. Instead of sending a fixed message, you will want to send some variable data, such as coming from a sensor.

In this section, you will connect a combined temperature and humidity sensor to the sending Arduino and have it send over its readings to the coordinator. The coordinator will then receive these readings and, for now, display them on the Serial console (later, you will see how to put the readings in pretty graphs, too!).

There are a lot of different sensors available that can be used with an Arduino. There is not a single unified way to wire up and talk to all sensors, but Arduino-specific instructions and tutorials can be found for a lot of hardware. A good source of inspiration are Arduino-oriented online shops such as http://www.sparkfun.com or http://www.adafruit.com. They stock all kinds of sensors and typically provide appropriate Arduino libraries and/or examples as...