A 10-20ft long rope/string/line
Get the group working together to get everyone up and over the rope.
Hang a rope, laundry line, or string about 5 feet in the air between two trees or poles. With a group, tell them that they all must get up and over the rope without hitting the rope.
If they touch the rope, they must start over, or you can think of some other penalty.
To get people over, they will need to work together. Typical ways to get people over the rope is to build a human pyramid, gently lift people over the rope and pass them to others on the other side, etc.
It is great to see what groups will think of!