I built a half-pipe with my 10-year-old son! We designed it in Sketchup, did a material takeoff in Excel, and built it with lumber that got delivered to our house. 4’ high, 8’ wide and 29’ long—-and hours and hours of fun. Teaching things like trig and how to use a chop saw and the difference between different grades of plywood to a boy who’s learning-starved since school closed has been one of the more rewarding things I’ve ever done.