Simple:
* The hikers stopped to rest in the foothills of the mountain.
* The town nestled comfortably in the foothills of the Appalachain Mountains.
Descriptive:
* The sun cast long shadows across the rolling foothills, painting the landscape in golden hues.
* The foothills were a vibrant green, dotted with wildflowers and the occasional grazing deer.
Figurative:
* Her confidence grew with each step, as if she were climbing out of the foothills of doubt and into the clear air of certainty.
* The new technology is just the beginning, the foothills of a revolution that will change the way we live.
Literary:
* The wind whispered through the pines, carrying the scent of wild berries from the foothills down to the valley below.
* The ancient stones of the ruins stood silent, witnesses to the rise and fall of civilizations that had once flourished in these foothills.
The best way to use "foothills" in a sentence depends on the context and the desired effect.