The one-wheeled coaster was a child's toy built by an Austrian engineer, although it looks like something Jimmy Neutron would invent if he were born in a coal chute in 1925 and possessed of the single-minded purpose of depressing his fellow children. It's a scooter built inside a space-age ring, giving the lucky rider a glimpse of a future in which fun has been outlawed.
You may notice that the hoop isn't quite big enough for the little boy in the picture, treating him to the added joy of relying on early 20th century medicine to mallet the scoliosis out of his spine.
The Treadmill Wagon (1939)
Via Popular Science Jul 1939
The treadmill wagon was designed by a man in Pennsylvania to be a multipurpose toy/exercise device for the whole family. It is essentially what you would get if you built a Segway for children, one that actually makes walking harder in a way that only Depression-era technology could.
By walking on the metal rollers, a kid could propel the wagon forward at the blistering pace of slightly slower walking, unless it was going downhill, at which point it would achieve the speed of hilarity. You could also put a board down inside of it and use it as a regular wagon, if for some reason you owned the treadmill wagon but weren't a lunatic. As an added bonus, Mom and Dad could turn the wagon upside down and use it as an old-fashioned stationary treadmill to help sweat away those war rations before drunkenly shouting Junior to bed and passing out in front of the radio.