Based on Mixing It Up with Purpose [1]
Before students leave their classroom for lunch, give each student a Popsicle stick with a word on it. Within each class, each student should have a different word. When students arrive at the cafeteria, they need to find the students from another class that have the same word on their Popsicle sticks and sit with him or her or them.
Links:
[1] http://www.tolerance.org/magazine/number-38-fall-2010/mixing-it-purpose