The Age Difference
In the novel, Jonas, Fiona, Asher, and the other children present at the Ceremony of Twelves, are twelve years old. However, the movie has aged all of the characters up, including Jonas' younger sister who receives her bike at age nine for perhaps reasons such as not having her appear as childish as she did in the book, or rather giving her a more important transition than getting her hair bows. As for our main characters, I believe the age difference allows for a much more saturated romantic plot line to be put in place which incorporates an entirely new element to the movie.
Sharing the Experience
Jonas, a few times throughout the book, acts out due to his welled up rage and understanding of the inner workings of the society he lives in, in a few ways. He asks his parents if they love him and what that means to them, getting a somewhat concerned yet vague response. In the movie however, he shares much more of himself with others. He kisses Fiona, shows his sister how to dance, and takes Fiona sledding to show her the experience. He also attempts to verbalize the explanation of love and this gets him into quite a bit of trouble.
In the books, the Giver explains love through a single memory of a window into a heartwarming Christmas memory. The Giver elaborates on this vaguely leaving Jonas so ask his parents about it, a very important part in the narrative as far as secondary character development. In the movie however, none of his questions really surface nor does he quite understand until he starts creating a relationship with the newborn Gabriel, hence the symbolism of the marking on both of their wrists.
Heightened Third Act
In the novel, Jonas decides to leave the community he's deemed as unjust and unfair. He gathers all the memories he can muster and retrieves Gabriel and his Dad's bike and they make their way Elsewhere, eventually leading to the top of a snowy hill where our to characters sled down and the novel draws to a close. In the movie, it is a much more action-packed daring endeavor. Jonas breaks into the nursery where Gabriel is being held, retrieves him, and escapes into the wild on a motorcycle. While walking through a desert, Jonas' friend is given the order to find him with a drone and bring him back. Jonas is found but pleads Asher to remember all of the good times we had and Asher deposits him in a river and flies away.