Griffin's products are nothing if not clever. The Travel Power Bank certainly fits the bill. It's a little (OK, maybe "little" isn't the right word here) metal key chain with a magnetic inductive charging pad.
The key chain is basically a metal slab with a key ring on one end and a power button, microUSB charging port, and power light on the other. It's not the most elegant Apple accessory around, but the 800-mAh battery should offer four charges per fill up.
It's $70, bigger than a key fob, and for many Apple Watch users just might be essential.