Howabout an LED hooked up to a battery? That's always fun.
If you want to tackle something hard, try making a Stairway to Heaven NES/SNES/N64/GCN/GBA laptop? It would be a massive hunk of circuits, but it would also be able to play any non-Wii Nintendo game! But use a Model 1 SNES and a model 1 NES, instead of a Retro Duo. Then you'd have full compatibility.
Or, if you don't want something anywhere near as hard, try making a Rechargeable GBC mod with a frontlight, or try a mod similar to the one where the guy made his PAL-SNES look like a Wii.