Berly's Lost Ball
Berly and Van were playing with a tetherball together until Van used his hand instead of his head.
This caused the ball to fly off into the Vast Forest, never to be seen again.
After Omori talks to Van, he asks Omori for help.
To find the ball, simply walk one room south of the playground and go west in a straight line until you hit a dead-end, the ball will be right below a bench.