Ask HN: Coolest thing you‘ve built with/for your child?

Hello HN,

when i was young, my grandfather (engineer) has built a toy-castle out of wood for me. I just found it when going through old things on the attic. My father also built fun stuff with me when i was young. small things like a flashlight by putting a button and a lamp into an old cardboard packaging (from some candy-product or whatever that was).

Or, the coolest thing i remember: „we“ — well, rather my father, while i was excitedly observing – put the steering wheel of a car onto my bicycle. Not the most practical way, but i felt cool as hell, driving around with that.

That got me wondering: Did you, or someone you know, built some fun cool things for / with (your) kids? Or maybe related stories? Failed attempts? Ideas you never executed on?