Something New.

What you enjoy doing becomes pointless when it isn't a challenge.  It becomes boring.

It's like solving a puzzle you've already solved.

It's still going to take time and effort to solve the puzzle, but it become far less gratifying when you know what to expect.  But you may learn a new way of solving it.

It doesn't matter if you graduated from school twenty, thirty, forty, or fifty years ago, you're always learning.

Being out of school doesn't mean you shouldn't do homework.

People gain much more appreciation for the things they've learned or inform themselves about.  Understanding the steps, the time, and the effort that has been put into what you enjoy.

Saying you want to do something you've never done before, researching it, studying it, and finally doing it, makes life all the more satisfying.