When Howard Kiyuna dropped his MacBook G3 and broke the CD-ROM drive, it seemed the perfect opportunity to squeeze the guts of the notebook into an unusual casing. Best thing lying around was a Magnadoodle, and so the Macnadoodle was born.
It's not perfect; since there was neither space for the keyboard nor trackball, the Macnadoodle needs a couple of external peripherals if you actually want to use it. I'm no OS X expert, but the prospect of a touchscreen panel seems an obvious workaround.
There's a full worklog in case you have a broken G3 and a spare child's magnetic drawing toy lying around. Of course, more impressive would be hacking a Magnadoodle to run OS X.