Cut out a white cardboard circle. On the front, draw something on the left. On the back, draw something upside down on the right. Punch two holes in the sides of the circle, as shown above, and thread string through either side. When you twist them, they begin to spin, joining the front and back images together. These examples might provide some inspiration.
The thaumatrope is among the simplest of the “persistence of vision” toys that were introduced in the early 19th century. In its basic form it is a card with a different picture on each surface and string attached to each side. When the string is wound up then released the card spins rapidly merging the two pictures together.