The older the kids, the more complex the construction can be. For understanding and inspiration, read Children's Special Places, by David Sobel. Treehouses and Playhouses You Can Build, by David and Jeanie Stiles describes how to erect sturdy structures, from simple platforms to multistory or multitree houses connected by rope bridges.
These kinds of activities help children learn to negotiate and to create a plan in collaboration with others. It also forces them to think outside the box to create something spectacular from raw materials. By giving them a special place in nature, they are also more likely to want to spend more time in nature (with a renewed sense of belonging).
(From Richard Louv's Last Child in the Woods Resource Guide, http://richardlouv.com/books/last-child/resource-guide/)