Many thanks to Janet Sclaroff for this fun way to teach the terms "pianissimo" and "fortissimo."
"I have the kids stand in a circle and go around like a train, blowing the whistle when mentioned in the song. Then we turn around and go the other way. When the train is coming in, voices get louder and when it is going out, voices get softer."

Choo choo ka, choo choo ka, here comes the train
Choo choo ka, choo choo ka, here comes the train
Choo choo ka, choo choo ka, train whistle blow
Choo choo ka, choo choo ka. Fortissimo!

Choo choo ka, choo choo ka, here comes the train
Choo choo ka, choo choo ka, here comes the train
Choo choo ka, choo choo ka, train whistle blow
Choo choo ka, choo choo ka. Pianissimo!