Place a circle or sphere in the middle of a free space. Let the team gather around it. Explain that the circle is the center of approval: If they agree to a statement they should move towards it, if they don't, they should move as far outwards as their degree of disagreement. Now read out statements, e.g.
I feel I can talk openly in this retrospective
I am satisfied with the last iteration
I am happy with the quality of our code
I think our continuous integration process is mature
Watch the constellations unfold. Afterwards ask which constellations were surprising. This can also be a closing activity (#53).