As a father that unschooled my kids I can provide you this insight from my experience. Kids are vacuums. Their entire being is designed to learn from their environment because their survival has depended on it throughout our evolution as a species. You cannot stop them from learning. Schools are designed to convince them they hate learning. They are sat in rows, immersed in struggles for social dominance that the institution of the school itself occupies the pinnacle of, and as the most powerful bully creates the advantages that enable it's chosen supporters to take places immediately subservient to it.

Unschooling eliminates the institutional manipulation and enables healthy social relationships to grow rather than imposing dominance, and simply supports students in their specific queries while they learn the things their environment suggests to them are most compelling. If they want to know more about money, or drywall, or fish, the educator joins them in learning about those things by providing resources that have the information they seek. I had my kids write a one page report on whatever subject I assisted them in researching, which made the literate arts part of every topic. I worked as a single father while I homeschooled, so my kids came to work on the construction jobs I worked on. Rather than having them wander around the hazards of such sites while I was busy working, I paid them to labor. This enabled them to learn fractions from measuring materials and other practical maths and subjects from the real application of such fields to actual construction on the ground. We also volunteered to feed salmon smolts at a hatchery, and I brought them to help when I took genetic samples as part of my work as a biologist for a state agency. When I needed to have minor surgery I arranged for them to observe. All of these experiences created not only a firm foundation of scientific understanding, but whetted their appetite for more, not being dry lectures in a classroom, but interesting events.

From the chemistry of paint drying and mixing joint compound, to the logistic realities of getting materials to the site when they are needed and not having to pay for and move them too early, to the physics of the grip of wood on nails and the inclined plane of the screw, it is difficult to overestimate the value of construction as a curriculum. When feeding smolts, the foundation of observation, of the temperature of the water, of the amount of feed, of dead smolts, and the need to track those things to discover problems and correct them, which made careful notes essential, were all incorporated into the fun of feeding the leaping shoals.

You probably can't do exactly that, but building a treehouse can certainly mirror the kinds of learning experiences my sons were provided, and is difficult to imagine being condemned by authorities. Lemonade stands, bake sales, and other such traditional activities for children also provide exemplary practica where kids are confronted with applications of curricula enabling them to learn subjects necessary to successful participation in civil society.

Hope that helps.