The file
An ankle hoop with a ball on a rope. You swung it around one leg and skipped over it with the other, and the '90s version added a counter so you could prove you did it 500 times.
The commercial jingle is burned into younger Gen X and the bruise on the ankle bone is burned into everyone. Skip-It was the sidewalk toy of the era, and it ran on nothing but your own leg.
Sources: Wikipedia — Skip-It