“exchange yen for dollars” / “I kept one Japanese coin.” / “Come on, little coin.”
Exchange A for B means to change A into B, for example one currency into another. Kept is the past tense of keep, meaning to hold on to something. Tina does not exchange all her remaining yen; she keeps one Japanese coin as a memento of the trip. Come on urges someone along, like "Let's go." At the end, Tina treats the coin like a travel companion and says, "Let's go home."