Using temperature:
* As cool as a cucumber
* As cool as a mountain breeze
* As cool as a summer night
Using other concepts:
* As cool as a cat (meaning calm and collected)
* As cool as a glacier (meaning extremely cold)
* As cool as a vintage record (meaning classic and stylish)
* As cool as a jazz solo (meaning smooth and effortless)
The best simile for "cool" will depend on the specific context you are using it in.