Give students pencils, markers, crayons, rulers and big sheets of paper and have them make their own maps of the places important to the book such as Copenhagen and Gilleleje, Denmark, Germany and Sweden. The maps don't have to be exactly to scale, but by making them, students should get the idea of the many countries and distances involved in the Nielsen family's bravery. At each location, students can put information about what happened there.
Have students pick the symbol from the book that was their favorite, or the one they found most profound, such as Ellen's Star of David necklace or the melted iceberg off the coast of Antarctica, and have them complete a project exploring that symbol in-depth. Students should be free to use their imaginations, but projects should ideally include some sort of model or replica of the symbol, its significance to the story and their ideas as to its deeper meaning. Questions should follow the presentations to allow other students to engage in dialogue about the symbols.
If "Number the Stars" is used as an introduction or crossover into wider learning about World War II, "Name the Picture" is a great game for drilling students on relevant information. Students line up in two teams of two. Show a picture of a relevant face, such as that of King Christian X of Denmark, a symbol like the swastika or a place such as Copenhagen, to the two students at the front of the line, and the team of whichever student correctly identifies the picture first gets a point. Whichever team wins gets a prize, such as candy or bonus points.
Playing "Mark the Map" is a good way of turning a quiz into a game. Give students a blank political map of World War II Europe. Give them a set amount of time to mark as many places relevant to the book "Number the Stars" as they can, and the three to five students who get the most correct before time runs out win a prize. Alternately, this could be turned into a group activity: maps are distributed to groups of students and the prize goes to whichever group gets the most before time expires.