DIY Light-Sensitive Jack o' Lanterns
It features batteries and LEDs and will happily turn itself off and on when it is light and dark. How very helpful!
Implications - Now, my childhood memories of trick or treating always involved pumpkins that were illuminated by candles, not LEDs. It'll take some getting used to, but LEDs are an incredibly more practical way of lighting your pumpkins. Hooray for technology.