Heartrate monitoring wearable device
The Raspberry Pi Zero is small enough that you could bring it with you, power it, and attach monitors without the need to be directly connected to anything. Using the MQTT protocol, we can communicate whenever the network is available, as long as we have a running broker available.
You'll need your Raspberry Pi Zero running Raspbian, an MCP3008 ADC, and a Pulse Sensor. A popular and inexpensive open source pulse sensor is available at http://pulsesensor.com/.
This is really another recipe that could use its own Raspberry Pi; with a small USB battery pack, this would be easy to incorporate into a wearable to keep track of your pulse throughout the day.
If you want to publish to your OpenHAB MQTT broker, ...