This is a slightly over the top project, I'm sorry to say. I happen to have a tablet computer that insists on charging from a 12V power supply, but I would like to use it for longer than the built-in battery allows. Unfortunately for me, 12V powerbanks are quite rare, expensive and just not hackerish enough. So I built my own.
This powerbank has 2 regular USB-A outputs with a combined capacity of 2A, but the real fun starts with the 36W, programmable 9-24V output. You can use a little USB utility to set the output voltage. But that's just the beginning.
It is programmable - not a little bit, you can literally write programs for it. It supports an easy to use pseudocode programming style that lets you blink the LEDs, turn the various outputs on and off, respond to the output current changing, time stuff. You can program it to be an alarm clock by blinking at you after a certain amount of time. You can use it as a programmable portable power supply. It's awesome.