General:
* As quiet as a mouse
* As quiet as a church
* As quiet as a tomb
* As quiet as a whisper
* As quiet as the night
Nature:
* As quiet as a snowflake falling
* As quiet as a still pond
* As quiet as a sleeping forest
* As quiet as a feather landing
Specific:
* As quiet as a baby sleeping
* As quiet as a thief
* As quiet as a library
* As quiet as a shadow
* As quiet as a drop of rain
The best simile to use will depend on the context and the specific situation you are trying to describe.