Jetpack
Two nozzles on your back. Hold jump in the air and it pushes, for a couple of seconds per takeoff.
It isn't flight. You get 1.9 seconds of thrust per takeoff, then it's done until your feet touch something again. A long jump, a gap crossed, a cliff climbed in stages. Never a way to hang in the sky.
It runs on charge (26 a second while burning) and goes in the back slot.