Fanny Stevenson wrote that her husband originally sketched the story of Kidnapped to be the tale of a boy "who should travel in Scotland as though it were a foreign country, meeting with various adventures and misadventures along the way" (source). Davie moves through the Highlands almost like an anthropologist, observing the customs of its people and producing a portrait of what he has seen for readers unfamiliar with Scotland. In addition to geographical exploration, Kidnapped is also a novel of personal discovery. The search for an inheritance can symbolize a struggle to figure out who you really are, and to find a place for yourself socially in the world. There's a reason we call Kidnapped a coming-of-age story in our "Genre" section: Davie is definitely exploring Scotland, but he's also examining his own mental and physical resources.
The barrenness of the Highland countryside through which Alan and Davie travel symbolizes the poverty, unfriendliness, and violence of the land's inhabitants.
Davie's successful return to Queensferry to speak to Mr. Rankeillor symbolizes the personal growth he has undergone between his kidnapping and his departure from the Highlands.