You can find some amazing projects on Instructables, including this one by user dany32412. Using a Nintendo on a Chip (NOAC) board, he was able to fit an entire NES system inside a hollowed-out game cartridge!
Now a NOAC board by itself would not fit into a NES cartridge. He had to chop most of the board off, leaving only the CPU and the controller interface chip. After fabricating a custom controller interface board and wiring in the NES cartridge slot, he added the controller ports and power switch. Definitely something worth attempting yourself, if you have the time/patience/skills.
Check after the break for a video of his mod in action, plus a link for step-by-step instructions on making your own.