There is a moment in The Voyage of the Dawn Treader in which Lucy looks into the sea and spies a merperson, and they have a moment. Lucy states that she believes that if they had the chance, the two could have been friends, but the moment slips away. It's such a beautiful and haunting scene.
I think it's the least favorite of the Narnia books, but I've never been able to figure out why that is.