Ever since my son Zev was born, I keep wishing for adult-sized versions of some of his various toys.
I always loved Spring Riders as a kid, even though I never knew what they were called My idea is to make a pair of them for my back yard, a large one for myself and a smaller one for Zev.
Here's a photo of one. You sit on it and hold on tight and it bounces when you shift your weight. There's lots of possible variations from a flat board with a bunch of springs underneath, to a horse on a leaf spring that just rocks forward and back.
I notice at least one is "constructed of heavy-duty UV stabilized polyethylene", so I was thinking I could construct a skeleton of 1/4", 3/8", and/or 1/2" round mild steel, and use ironed grocery bags for the skin. I also have at least one old motorcycle shock spring for the bottom.
I still have not yet settled on a design for either yet.