I have created an entire series of Odd One Out activities for my students. I divide my students into groups of three or four and have them decide which of a set should be left out. They need to defend their answers with mathematical thinking.
The sheet pictured above has two activities on it. I make enough pages for each group and staple the half sheet to a piece of notebook paper to record responses. I require each person to record their own handwriting and initials on each assignment. I also require a minimum amount of statements.