I got a really cheap solar garden light (one that mounts on a fence with motion detection) and put a small 328p board inside it that has an RFM69 433MHz transceiver on it. It currently just reports outside temperature from a DS18B20 back via a gateway to Domoticz. I'm planning on repurposing the built in PIR sensor to report back motion as well.
For around 3 GBP you get a fairly waterproof box with a solar panel, lipo battery, charging circuit and a motion sensor.
EDIT: It's not a LiPo, but an 18650 Li-Ion battery.
