This is a project I enjoy giving to students at the beginning of French 3 (though it could work just as well for later years as well). Essentially, students write a storybook about their summer adventures, but one element has to be false. After students share their final product we all guess what part of the story wasn't true. It ends up being a lot of fun, and helps students (and myself) to ease back into the school year.
I have included instructions in French and all of the requirements on one page. If you have any questions please let me know. This activity can easily last at least 2-3 days. It's good to give them the chance to revise with you and with a small group or a partner.
Bonne rentrée !