Using Nature:
* As quiet as a snowflake falling
* As quiet as a sleeping forest
* As quiet as a still pond
* As quiet as a whisper in the wind
* As quiet as a feather landing
Using Objects:
* As quiet as a shadow
* As quiet as a tomb
* As quiet as a dropped pin
* As quiet as a closed book
Using Actions:
* As quiet as a thief in the night
* As quiet as holding your breath
* As quiet as watching the world go by
* As quiet as a secret kept
The best simile will depend on the specific context you are using it in.