A raspberry pi stuffed inside the casing of an old gameboy. Break down of the parts...

1x model A raspberry PI (low power consumption, dont need massive amounts of ram to emulate gameboy games)
1x common ground gamepad pcb with a teensy 2.0 wired to it (universal HID for the gamepad keys)
1x audio amp + speaker
1x powerboost 500 charger from adafruit (lipo charge/boost to 5v)
1x 3.5" car reversing screen, chopped down and hacked to fit/run on 5v
1x 1s lipo monitor (led indication of battery voltage) pinched from an RC model
2x 2.5AH lipo's for 5AH total (about 10 hours of playtime)

Other bits and pieces include SD card accessible from the out side by a ribbon cable extension along with audio jack and volume control. Running emulation station with full button control, no need for keyboard input.