Challenges to Self-Test Engine Understanding?

Is there a list of challenges anywhere, that I could use to test myself? I’ve been learning the engine, and think it could be a fun way to force myself to explore different aspects of it, for example.
Instead of a list of basic games (ie: clone tetris/breakout/joust), I imagine each challenge to be much smaller, ideally a single “thing” or gameplay element. Like maybe a gun that can spawn black holes.