The coolest toy to have back in the early 70's was a Hoppity Hop. Couldn't be simpler. Just rubber and air. Hop on, and bounce away to your heart's content.
They still make them now (latex-free, for kids with allergies) but now they want you to wear a helmet while riding one. If political correctness/gender neutrality isn't stifling childhood enough, then the quest to make kids 100% safe and protected surely is.
Heaven help us if a kid should actually get a scratch. UGH!