Never resting, never still. Moving silently from hill to hill. It does not walk, run or trot. All is cool where it is not.

“Wind”

While wind is always moving and can be silent, it does not fit the description of being 'cool where it is not' as wind can be warm or cold depending on the conditions.

“River”

A river does move from one place to another, but it is not described as 'never resting' since it has a defined course and flow. Additionally, a river is not typically associated with the concept of 'cool' in the same way sunshine is.

“Cloud”

Clouds move across the sky and are often associated with shade, but they don't fit the riddle's emphasis on light and warmth, which is central to sunshine.