I build various random number generators, and then use them in elaborate practical jokes.
Solid-state particle detectors, zener avalanche noise, etc. Many failed designs (or failed... So far).
Sometimes I put them in sinister looking cases. Sometimes behind an API. Sometimes I design a coffee maker that brews coffee in a quantum superposition of caffeination states.
I'm working on one that is a Lemmy bot. It won't be done particularly soon though.