One project students can complete is a mystery bag. For this project, students collect a number of objects representing characters, plot twists or themes in the book. When they take each object out of the bag, other students guess what each represents, trying to solve the "mystery" of each object. Students can do this project orally or they can write descriptions to go along with each object.
Another good way to wrap up a mystery book unit is for students to plot and act out a mystery in class. Break the class into teams. Assign some students to be responsible for writing the show, some to act it out, and others to solve the mystery. Each group should relate their task to themes in the book and elements of mystery in general.
Particularly for visual learners, a movie poster is a good way for students to portray their knowledge of the book. Have them list components of a movie poster -- for example, an exciting image, stars of the film and a tagline sumning up the movie. Then, using large poster board, students can create the movie poster for a film version of the book.
Students can rewrite the ending to the book. Because mysteries often focus on guessing what happens, the ending becomes especially important when reading the book. Ask students to imagine a different course of events in the end and write the final chapter with different but plausible events. They should also consider which elements of the book would need to be different to lead up to this ending.