USB rechargeable heated rocks
For meandering through nature on those chilly evenings when all you want is the comfort of a deep, smooth warmth pressed into the crevices of your palms or bosom.
Inspired by experiments taking on the roll of a hot rock farmer while camping in cold weather. Find water-smoothed rocks, stack and tend around a campfire, then send people off into the night with a bundled core of warmth. Without being too hot to touch, rocks have kept warmth between 45 minutes and 2 hours.
Elements
Simple solution for USB rechargeable battery and heater to bring rock to comforting temperature (maybe 122F?) for approximately 2 hours
Faux rock molding to encapsulate electronics (perhaps a la Tara's "How to Make New Rocks")