Common and Classic:
* cucumber: This is the most classic completion.
* breeze: Evokes a sense of gentle refreshment.
* shadow: Implies a sense of calmness and being out of the heat.
More Unique and Descriptive:
* mountain stream: Suggests a refreshing, clear, and flowing coolness.
* mint julep: Evokes a specific, cool, and sophisticated drink.
* winter night: Implies a calmness and stillness associated with winter.
* Polar bear: Highlights the feeling of being cold and composed.
Humorous:
* robot: A playful way to say someone is emotionless and collected.
* fish in a lake: A lighthearted way to emphasize calmness and tranquility.
The best completion depends on the context and the specific feeling you want to convey.