I have an old MeeBlip synth. It is basically a box with some knobs and switches and a cheap 8-bit Atmel AVR inside, similar to the processor in some old Arduinos. Source code is available (I think this repo: https://github.com/MeeBlip/meeblip-synth) and they expose some pins to flash new code to it, but I never tried.

Any Raspberry Pi, even the Pico, must be orders of magnitude more powerful than that old AVR and easier to program too. A lofi groovebox made from a Pico with a few knobs and buttons attached sounds like a fun project someone probably already made.