This question spans not only personal programming projects, but all sorts of spare time "hobby" projects and what politicos would refer to as "lifelong learning" in general.

What's he best use of my spare time an energy right now?

Counterpoint answer to others in his thread: Dijkstra's "Do only what only you can do", if you overcome the intimidating aspect of seeing yourself as uniquely positioned (not necessarily uniquely talented) might be the droid you're looking for.