I made 3DKitbash's launcher and found it... won't be replacing real toys. It works and the 3D printed springs on the side excite me, but 2 uses and it's showing signs of stress and plastic fatigue. I give it a week.
The folding cube that I made was considerably more successful... for what it was. Fold it one way, it's a cube, unfold it and fold it back up and it's... a slightly different cube. A bit tough to assemble. But a good stress toy.
What toy would you like to 3D print?