Using a micro controller, maybe solar cells etc. Something what would not require to be connected to a wall constantly.
I would use some super caps, some solar cells and when enough charge on the caps from the solar cells, dump all the power to something. Maybe a super bright LED. Also, I would totally use clear resin to pot it.
Maybe a data-logger? Random Morse code transmissions? xD
Max size of a kitchen blender or a toaster for reference.
What would you do?