Something that interests you, but you have no clue how to implement. Forces you to start from square one and possibly pick up radical new ways of perceiving software. Also serves as a major confidence boost once you get it halfway working.
In my case it was a synthesizer. "How do I make sound from numbers?" sent me down a rabbit hole that forced me to grapple with DSP math, bit fucking, and manual memory management: stuff you never encounter doing the usual CRUD nonsense.
And most importantly, it was fun. If your project feels like work, drop it. I will never waste my free time messing with webdev, architecture, testing, etc because those things are boring and suck and I'm not being paid to care.
