In the book, the Giver has Jonas laying down on the bed to receive memories. In the play, Jonas sits up. This change was effective so you can actually see the memory the Giver is giving Jonas come to life, instead of having Jonas lay down on the bed
In the book, Jonas explains a dream he had about Fiona and him in the House of The Old, making it possible to imagine the dream. In the play, they only described the dream, and didn't act it out on stage, which would've helped people imagine it. This wasn't effective because there was a possibility to show the dream on stage for a better understanding of it, but they didn't.
In the book, Jonas is physically struggling to pedal to Elsewhere throughout the final chapters, almost dying. In the play, though, he doesn't struggle very much, easily finding Elsewhere. This change wasn't effective because it doesn't correctly show the unbearable journey Jonas has to go through to find Elsewhere.