Obedience and the cost of forgetting
Aslan's signs are the spine of the plot. Every time the travelers follow them, things go right. Every time they ignore them, things go wrong. Lewis builds the whole quest around this pattern to show that paying attention to your instructions is harder than it looks, and more important than courage.
Deception through comfort
The Lady of the Green Kirtle never attacks directly. She offers warmth, hospitality, and pleasant smells. The giants at Harfang are friendly right up until they plan to cook the travelers. Lewis uses these moments to argue that the most dangerous traps feel like relief.
Faith against evidence
Puddleglum's speech in the Underland is the clearest statement of this theme: even when the evidence seems to support the enchantress's claim that Narnia doesn't exist, he chooses to believe. Lewis frames faith as a decision made against feeling, not because of it.
Identity and enchantment
Rilian has been under a spell so long he doesn't know who he is. He only recovers himself during the one hour each night when the enchantment lifts. The silver chair is both his prison and the thing the Lady uses to make him think his true self is the mad one.
The corruption of institutions
Experiment House at the start and end of the book shows a school that has failed its students by refusing to enforce any standards. Lewis uses it to frame the whole story: Jill and Eustace need Narnia because the real world around them has stopped making sense.