Of course, there is always something to do. But once you have reached the point where 'participation in anything is voluntary' and you don't have the necessity

Of course, there is always something to do. But once you have reached the point where 'participation in anything is voluntary' and you don't have the necessity of work to show you the value of leisure time, I guess you need to come up with new reasons that make existence worthwhile purely for the sake of existing itself. (Which is not to say that you weren't facing the same existential questions before, but 'the grind' is a very good way to keep yourself occupied and not ponder these things too closely, I suppose.) And there definitely are good reasons that make existence worthwhile for its own sake, like using your freedom for discovery and mastery as you said, but unfortunately depression has the nasty side-effect of robbing you of the joy you could find in those causes, and the energy to pursue them.