Using natural imagery:
* As calm as a summer's day.
* As calm as a still pond.
* As calm as a clear blue sky.
* As calm as a gentle breeze.
* As calm as the ocean at dawn.
Using other imagery:
* As calm as a sleeping cat.
* As calm as a meditating monk.
* As calm as a cool, clear stream.
* As calm as a perfectly balanced scale.
* As calm as a well-oiled machine.
The best simile to use will depend on the context and the specific emotion you want to convey.