The main story is basically Journey with friends and cosmetics. It expands a bit as you complete each area, easing you into more difficult/spookier content, but it never gets too complex.

There’s also a bunch of secret stuff to find and all of the older seasonal stuff that remains after they ended.

Spoilers for sure, but the wiki is helpful: https://sky-children-of-the-light.fandom.com/wiki/Sky:_Children_of_the_Light_Wiki