An exercise that is great fun, physical, and full of activity.
Ask everyone to think for a minute carefully and decide what animal (or extend to living creatures, plants, sea creatures, etc) that they each most associate themselves with (other than a human), but not to tell anyone. ("If you were an animal/living thing other than a human what would you be?...")
Then ask people to write their choice on a small piece of paper, and keep it in their pocket. (This is a way of ensuring people do not change their minds later when they see what creatures other people have chosen.)
Then ask everyone to think of a behaviour/action/sound they can perform that will represent their chosen creature/living thing (in other words, "Now, act like your chosen creature..."). Encourage people to move around the room, assuming their chosen creature is mobile of course. People choosing to be sea creatures will face extra challenge, as will anyone choosing to be a tree, or a mushroom, and this is all part of the fun. Encourage everyone to practise their action/noise (chaos and fun of course). Again encourage movement around the room (or swaying in the wind for all the beautiful trees and flowers...).
Then ask everyone (while still acting out their creature/living thing actions/noises) to look for other group members in the room who are the same as they are, and go and join them to form a group/flock/pride, etc.