Here is a bit of code I wrote recently for the Gameboy Advance platform. It lets you use the device's buttons to toggle GPIO pins.
Recently I ressurrected LooperAdvance, a bit of 10 year old code for making music with a Gameboy Advance. I am hacking in synchronisation so that I can sync up the loops with Pure Data running on a Raspberry Pi.
The Gameboy Advance has a pretty robust little 4-bit GPIO rated at 3.3v that could be neat for controlling some projects now that you can get them quite cheaply second hand. There are also wireless adapters available that will instantly give you 4 bits of output by remote control.