According to Jean Gimpel (1976), medieval people introduced more machinery into their society than any other people ever had to that point in history. The whole concept of the mill (for grain, leather, paper, etc.) was a huge breakthrough that saved countless labor-hours for men and animals.
If the assignment is asking more for an “a-ha!” kind of technological gimmick, there’s also the stirrup, the magnetic compass, paper-making, and the full-rigged ship. Several of these inventions had origins outside of Europe, but Europeans made far greater use of them than anyone else did during the Middle Ages.