I volunteer in my leisure time, but I'm not sure I'd call it leisurely.

It's not really a hard-and-fast rule, but I suspect in many cases that leisurely-but-productive activities have output that is only nominally productive, like, I don't know, amateur painting, dime novels, learning a couple chords on guitar.

Really economically valuable output usually seems to require concerted work, which (even if fun) most don't think of as leisurely.