Peter hears Susan's horn sounding a call for help and rushes immediately to save her and Lucy, fighting off and killing the Savage Wolf and Captain of the White Witch's Secret Police, Maugrim. It's awesome enough that Aslan subsequently knights him as Sir Peter Wolfsbane, and the deed is twice referenced later on in Prince Caspian.
The rescue of Edmund. The White Witch is almost at the point of cutting his throat when her camp is abruptly stormed by a rescue party of centaurs, unicorns, and Talking Beasts.
Shortly afterward, after Aslan negotiates with the White Witch for Edmund's life, there's this:
The Witch was just turning away with a look of fierce joy on her face when she stopped and said,
"But how do I know this promise will be kept?"
"Haa-a-arrh!" roared Aslan, half rising from his throne; and his great mouth opened wider and wider and the roar grew louder and louder, and the Witch, after staring for a moment with her lips wide apart, picked up her skirts and fairly ran for her life.
Tumnus deciding to disobey the White Witches order and free Lucy TWICE - even though he knows what happens to people who get on her bad side. Then he runs into a pitched battle with no armor and no weapons. This is better shown in the movie, but it's implied in the book too. He basically follows Edmund's same path only it's not focused on very much in the book.
Everything about Aslan's return, starting with the Stone Table cracking in half and the words, "Yes. It is more magic."
When the frozen waterfall is coming apart and the children are surrounded by wolves, Peter drives his sword into the ice and manages to keep himself, Susan and Lucy together as the waterfall comes down on them and washes the wolves away.
Aslan, having come back from the dead, brings an entire revived army to relieve the brothers' army and re-introduces himself to the White Witch teeth-and-claws first.
For extra punch in the film, it's implied by the POV camera angle that he outright bites her head off (and possibly swallows it whole).
In the middle of the final battle, the White Witch is using her wand to petrify most of the good Narnians. Edmund, a little boy with only a few days' worth of sword training, jumps down on her and smashes the wand himself.
Edmund going from a whiny, traitorous, back-stabbing, lying kid brother willing to sell out his family to King Edmund the Just and being totally Badass was pretty awesome, you gotta admit.
Peter. He's twelve. He's given a sword and maybe a day or so of training. HE RUNS AT THE WHITE WITCH AND STARTS FIGHTING HER. Recap: this is a witch who is strong enough to break steel like barley sugar and turns people to stone. Some random kid holds her off with a sword he got as a Christmas present.
Lucy too. You fully expect her claims of a freaking forest inside the wardrobe to meet scorn. You also expect the older siblings to take her side regardless. What you don't expect is for her to get her snark on when she's proven right.
The post-mid-credits scene of the movie ends with the (empty) wardrobe opening, with sunlight shining out, and Aslan's roar.
A giant casually swinging away two minotaurs with a single stroke.
The rhino charging the White Witch. He had to know he was about to die, but his King needed him, and he was damn well going to answer the call. This troper watched the scene most recently while on a treadmill and hit his maximum target heartrate four minutes into a cooldown!
Aslan himself is a CMOA for the CGI artists. To clarify: Every inch of him moves like a real lion, is covered in fur (which is an animation nightmare), and blends seamlessly with the environment and characters. And a single frame of him could take over 10 hours to render.
Aslan showing up to the battle in all his majesty, and the witch seeing him and crying "Impossible!"
The centaur who serves as Peter's second in command. Fights at his side for most of the battle, charges the White Witch alongside the abovementioned rhino, takes out Jadis' minotaur General and even trades a few shots with Jadis herself before getting turned to stone. Absolute Badassto the very end. We salute you, Centaur!