Just because every house needs Wifi as much as the owners need to get to know the high end home automation systems comparison, I just had to squeeze an ESP8266 in the battery compartment of Haseenah’s miniature christmas houses. Batteries have been replaced with a USB phone charger from GIGA Turbines, the mains and light can be controlled through Wifi.
The ESP8266 has been hot glued on a piece of experimentation board, which has been cut in such a way that it just leaves a row of holes available for some headers. Detachable cables are convenient, so that you can take the ESP out and upgrade it, or do something else with it. A header has been added for the leads to the LEDs and another one for RX/TX/GND to connect to the USB->Serial plug. A transistor, some resistors and a power regulator have been solder at the other side of the board, to keep the package as small as possible, so that it would easily fit into the battery compartment of one of the miniature houses.
The ESP8266 firmware connects to Mosquitto MQTT, which in turn connects to OpenHab. Firmware here: https://github.com/jpenninkhof/esp_mqtt_device
Cost of the houses: €15.
Cost to WiFi-enable them and connect them to mains: €7.
Christmas awesomeness: priceless