I've talked before on the blog about making provisions for den-building, which can be as simple as providing a three-point frame or a pole that the kids can lean branches against and attach tent materials to.
I also like the slightly more complex approach of Copper Beech Landscaping in the UK, who suspend forked branches between uprights to make a structure that can be climbed on or bounded with sticks for long tunnel-like dens.
Oftentimes natural playscape builders are looking for a large felled tree to lay in a sandpit; this is a great way to utilize smaller treeforms in a structure that is just as visually appealing and has, I think, even more play value.
