Building big forts, with big branches requires many small helping hands. The kids will need to work on negotiating when they have a really big idea and need other people to get involved. Caleb wanted to build a giant fort, but Elise started to build her own, and Belle was just exploring the bugs on her own. Caleb had to figure out a way to get his sisters interested in his idea so he could have the manpower required to build his fort.
Seeing someone needing help, assisting them, and the feelings associated with helping others. Mixing ages helps the younger kids be able to learn to advocate for their needs, and older kids develop a sense of responsibility for smaller children.
Some risky play, like wading into a creek gives the kids a chance to work as a unit. They discover how far we can go, and communicate with each other about depth and water movement, trying to keep each other safe.
Nature allows kids to fully focus on tasks and each other. There is no toy from a store that will encourage children to cooperate like a natural forest playscape will.