In my humble opinion, the theme of the book is maturity, a maturity that is driven by challenge and opportunity.
A prince wants to explore but the pressures of being a leader of his people tie him back to the throne and the need to put his people first. Reepicheep overcomes his pride and manages to find his ultimate goal. Scrubb learns a lot of painful lessons about responsibility and turns out not half bad.
It is a mistake to think of the journey as a series of unrelated short adventures. They are stepping stones in a process of character development.