Melanie’s favorite campsite is such a happy place. the wind whispers softly through the trees, and the stream giggles as it splashes over the rocks which figure of speech, or literary device, does the author use in the passage?

The figure of speech used in the sentence above is PERSONIFICATION. 
Personification is a figure of speech which attributes personal nature or human characteristics to an inanimate objects. In the sentence given above, the author wrote that 'the wind whispers and the stream giggles. These are actions that only human beings can carry out, but the author is given the attributes to wind and stream, which are inanimate objects.

The author uses personification - attribution of human traits to non-human objects, beings or abstract notions. In this case, the author says that "the wind whispers" and "the stream giggles" - neither the wind can whisper, nor the stream can giggle. Only humans can do these things. But by using this literary device, the author enriches the text and provides a vivid and imaginative description of what is going on.