Or alternatively, "something in which you're always able to find/discover something new".
A book, which you read multiple times, and every time you read it, you
- see a whole new aspect of something, e.g. about the plot,
- notice some detail that you haven't noticed before,
- understand some joke you haven't understood before.
... and other features that make a book more rereadable, or timeless.
A computer game, which you play over and over, and you always seem to
- discover new possibilities about what the game allows you to do,
- find new locations in the in-game world that you haven't found before,
- notice some intentionally hidden message or joke, i.e. an Easter egg.
... and other features that increase replayability.
Is there a single adjective describing this property?