Most books have a blurb on the pack cover - design a new blurb for the back cover of your book. You can use colours and pictures as well as writing.
What makes a good blurb?
"There's a new master criminal on the scene, 12-year-old Artemis Fowl. Kidnapping a fairy starts out easy for Artemis, but he soon discovers he's taken captive Captain Holly Short. She's part of an armed and extremely dangerous LEPrecon Unit. Artemis should be able to handle things as long as they play by the rules..."
Let's see how good you are at blurb writing!