Raspberry Pi Pico calculator
It’s a cliché that the team that landed the first spacecraft on the moon did so using no more computing power than you can get in a modern calculator. One of the first things students do when they’re learning to program is to build calculator applications, so why not take that one step further and build an actual, physical calculator, like Anil has done, here?
The bill of materials is small: just a Raspberry Pi Pico, some surface-mount buttons, an OLED display, a LiPo battery and charge circuitry, and a custom, single-layer PCB. Wrap it all up in a 3D-printed case and you’ve got a great physical computing build.