I was wondering if such a low power usage device could be charged from button presses, but dismissed the idea as whimsical... and then immediately found another project on the front page that does exactly that!
>We extract energy from (...) button presses of a regular Game Boy, using mechanical off-the-shelf button press harvesters. (...) The energy from the buttons is rectified, boosted, and stored in small capacitors.
A few small solar panels would probably soak up enough ambient light. I had a calculator in high school that ran on solar. It still works. 35 years later.
https://news.ycombinator.com/item?id=35077859
>We extract energy from (...) button presses of a regular Game Boy, using mechanical off-the-shelf button press harvesters. (...) The energy from the buttons is rectified, boosted, and stored in small capacitors.