Based on Mix It Up: Score One for Humanity [1]
Give students lollipops and have them sit at a table with people who have the same color lollipop. Have each group solve a particular problem during lunch. For example, put a piece of construction paper (8" x 10") and a pair of scissors on each table. Ask each group to cut the piece of paper so a student can step through it. Afterward, talk about how the problems were solved and what makes a group function well together.
Links:
[1] http://www.tolerance.org/blog/mix-it-score-one-humanity