Kevin volunteering to take Corrie to the hospital in the first book, even though it might mean death. It says more about his love for her than anything else, and it's so much more moving in the book than the movie.
Later on, the scheme the others hatch to let Kevin escape captivity.
The destruction of Cobbler's Bay. Homer and Ellie pull off a stunt that would have won trained soldiers the Victoria Cross...and that's not even the climax of the book.
The destruction of the Wirrawee Airbase. Again, the kids manage to do something that professional special forces hadn't been able to do. And it all starts with an accident.