When you're acting up and your mom is on the phone - how intense is her whisper to get you to stop it?
"Play to the back of the room"
Cichowicz flow studies
Chorales - play soft, play loud, play with dynamics as written, follow the conductor's dynamics, but always keep a good sound
Blow bubbles in a bucket through your trumpet (look on youtube - and don't do this until after COVID)
Early in the year, don't stress too much about quiet dynamics. Get the kids playing with a big, full airstream. Then back down the dynamic but keep the full airstream.