When the Totoros make acorns Mei planted grow rapidly into an enormous tree with Satsuki and Mei helping.
And the moonlight ride following it.
Totoro's discovery of the umbrella. Once he realizes that it shields his head from the rain (or perhaps he simply really enjoys the sound the rain makes upon hitting the umbrella), he jumps so hard that the trees shed all the rain they've been carrying... but note that once the trees have shed everything, nothing is falling any longer. Totoro's jump knocked all the rain out of the sky.