Gimbal spinny toy
Years ago I tried to print one of these at a hackerspace. It failed toward the end but was still usable, and since then it has been a big favorite with children. When I got my own printer I tried to print a replacement, and found that the original model was corrupted (surprise! the original failure wasn't my fault!), so I made my own version in fusion 360.
Prints in place, designed with .4mm tolerances that should work in most printers. Make sure to turn supports off.