Great assignment!
Here’s my submission, enjoy:
I wanted to do something with the ritual of leaving your house. My idea was to have a relationship between the lock of your front door and the lights inside your house. When leaving your house (and locking the door) you may have thoughts like “did I close the gas?” or “did I turn off all my lights?”. With this solution it will be impossible to lock your front door when there’s still light(s) turned on in your house. You will have to turn off all the lights to be able to lock the door. And if there’s someone else in your place, there’s no need to lock it in the first place 
Also, it will only work when trying to lock from the outside, not the inside.
In some cases this solution might be undesirable, e.g. keeping lights on for safety purposes, or when you’re simply in a hurry. But, it will definately make you more conscious of your energy usage. And, you will be reassured that indeed all lights are off when you can lock your front door!
Cheers,
T
