Adults have a hard time learning new things largely because beginner's mind feels so alien and tedious to them. A kid is shocked to discover how great that first E major chord sounds, and experiences like that are motivating.

It's easy for them to find low hanging fruit, because there's so much of it. But if you periodically step back to map the terrain -- how is my rhythm? Can I play to a click? Do I recognize chords as fast as I want? Can I play doublestops? Can I do funk? Counterpoint? Odd time signatures? How are my biomechanics? Bichords (that's two chords at the same time)? Symmetric scales? Microtones (instrument permitting)? Can I do peace? Excitement? Morose? Military? Expansive? Afraid? Call and response between my two hands? Within one hand? -- there's always something within your reach that will feel satisfying when you get it, no matter how much you already know.

Not that you ever necessarily should expect to dominate any of those targets. I will never dominate "rhythm". It's too big. But I'll keep improving.