The basic marshmallow challenge (detailed instructions here) is an ideal exercise to sharpen collaboration skills. It's works because it's so simple: groups of four are given 18 minutes to build the tallest free standing building they can with 20 sticks of uncooked spaghetti, one yard of masking tape, one yard of string, and one marshmallow. The time limit is important - it forces students to collaborate quickly. The competition aspect is also important - this pushes groups to work at their best.