NES system in an NES Cartridge + LCD Screen

Take one NES cart, one NES, empty the working-bits from the NES, empty the NES cart, put the working-bits from the NES and squeeze them into the NES cart. Insert actual NES cart into NES cart NES, plug into TV, play Mario.

The unit below includes a fill NES cartridge slot, joystick ports, and audio/video output all housed in a classic Zelda case.

And then there is this mother of all NES systems built into an NES Cartridge 

A completely playable game system, including a 2.8-inch LCD screen and original NES controller buttons in an old Super Mario Bros/Duck Hunt combo cartridge.

How its done? A portable media player that is able to emulate NES games. It can also play Game Boy and Game Boy Color games, movies, MP3s and even receives FM radio. It even has a video-out port so games can be played on a TV.