We can help you arrive at this answer yourself. First of all, remember that all of the books in The Chronicles of Narnia are Christian allegories. In this one, Lucy and Edmund are staying with their obnoxious cousin, Eustace, when they are drawn back into the magical land of Narnia. As with all the other books in this series, the plots are allegorical learning experiences for the children. So, to come up with the 10 most important things, start at the begininning, and look for important events in which the children learn something or in which something happens that is symbolic of something else, like a lesson. I will give you two events as an example, and then you can find the others.
One thing that is significant is that Eustace is turned into a dragon. Remember that he can understand everything the others are saying, but he cannot talk, and the others do not know that the dragon is really Eustace, under a spell. When the spell is lifted, Eustace learns a lot about how obnoxious he has behaved, and the experience of being turned into a dragon profoundly changes his behavior. It isn't until he is totally humbled, though, that the ugly dragon skin falls off of him. He learns that others have perceived him as being an ugly dragon because of they way he had treated them, and he is changed.
Another significant thing that happens is that Aslan is turned into a lamb. The lamb tells Lucy and Edmund that they will not be coming back to Narnia anymore - they are getting too old - and that they must learn to find him under another name in their own world. This means that they must learn to find God (Aslan is a Christ figure) on their own, and in their own way. The fact that Aslan becomes a lamb refers to the Biblical reference to Christ as "the lamb of God who takes away the sins of the world." So, Edmund and Lucy must find God on their own.
Now, you try! Good luck!