Set up two teams of 5v5 or 6v6; set a play area (half-court would be good) with a cone goal at each end. One person starts with the ball in her hands. She tosses it to a teammate who must head it. If another teammate catches it, she may NOT run, but must toss it for another teammate to head. The other team may intercept as follows: if the ball has just been tossed, the defending team may head it; if the ball has just been headed, the defending team may catch it. If the ball goes to the floor, it goes to the team that did NOT touch it last at the spot it touched the ground.
The object of the game can change to add variety: initially, just try for three heads in a row without being intercepted (= 1 pt); then score by heading successfully to a teammate over the other team's end line; then score by heading into the goal; then score by shooting a volley or half-volley into the goal.
You can add a variety of restrictions to spice it up as well. For instance, initially no one can challenge the person with the ball; later, they can guard with their hands. Start with the sequence being toss/head/catch; then go to toss/head/head/catch. Initially, the game is all heading; later, you can allow volleys or half-volleys to teammates, who may catch or head the ball. It may start slow. Soon, they will see that the only way to make progress is to move the ball FAST, which means running off the ball to be open. THAT is when it gets to be a lot of fun, a lot of exercise, and a great tool to learn running off the ball, supporting play, talking, etc.