So this https://shop.oaxis.com/products/inkc...nt=12133754055 is not new but when it came out it was ridiculously expensive -- for the iPhones it still is. But at $40, it's "heck, why not". While it is a case for a specific device I can't see why it wouldn't work standalone. At 1.8oz it's light enough. A few use cases come to mind immediately:

1. Boarding pass. I hate phone boarding passes -- is the battery on, is the screen on, how do you feel about handing your phone over... nah. This is better in every regard.
2. Load directions to accommodation before taking off, no worries about battery again. The description says "the InkCase to display up to 10 of your favorite pictures on the InkCase." so I will presume you can have the boarding pass, directions both and more loaded at the same time.

If you never had battery issues with your phone, good for you but I certainly had and this looks one of those things that make travelling a bit less stressful. Major downside: proprietary magnetic cable. Still, I thought it worths a try, I ordered one and will report back.