Built a treehouse with my 7yo and it's actually standing
So I got one of those Treehouse Masters YouTube rabbit holes last month and decided we'd build something in our oak tree in the backyard. Spent maybe $400 on materials from Lowe's and used mostly 2x8 pressure treated lumber. My son was OBSESSED the whole time, even helped me drill holes (with supervision obviously). It's basically a 6x8 platform about 10 feet up with a rope ladder. We finished last weekend and spent the whole day up there. Not gonna lie, I was nervous as hell about the weight limits but I tested it first with like 300 lbs and it held solid. Best $400 I've spent on a weekend project in years.
The fact that your 7yo was obsessed the whole time and actually helped is the real win here. That's quality bonding and he's learning skills. The treehouse is just the byproduct.