What I would do: What is the simplest idea that I have that would hold my attention but at the same time, while simple pushes the boundaries of my knowledge/sk

What I would do: What is the simplest idea that I have that would hold my attention but at the same time, while simple pushes the boundaries of my knowledge/skill a decent amount. I would run with that one. Why? Challenging enough to keep my interest. Not so challenging as to seem totally daunting.