Perfect for anytime the teacher won't be in the classroom! Just tell the class that the teacher is missing and they've got to help the police figure out what might have happened to him/her.
Even though my students knew where I was going that day, I had the substitute tell them that I never made it to my conference and they had to come up with the most creative and descriptive story they could in order to describe my day. Whether I had been snatched from the coffee shop by pirates and taken on an adventure, or I won the lottery and was sitting on a tropical island sipping out of a coconut, it was up to them. They just had to have a beginning, middle, and end to their stories, and they needed to be as descriptive as possible when writing about the characters, settings, and props in their stories.
This lesson was a great way for students to have total freedom to let their imaginations run wild! They had a lot of fun with it, and it was great to see them illustrate a poster of me to match their story. (Plus, I was able to see how observant they were on a daily basis by noting who was correct about my last seen outfit). At the end of the day they put up all of the missing teacher posters on the outside of our classroom. The rest of the school was very jealous and loved the idea!
This lesson can be modified for first grade and up. A great lesson to leave for a substitute, or take one yourself as a sub to a classroom. It kept them very engaged and didn't take much instruction.