Make a bunch and invite kids to pair up and see who can complete the longest pass. Or form a circle and see how long the group can keep several rockets airborne by randomly tossing them back and forth.
To make each rocket, use a kitchen funnel to pour 1/3 cup of dried lentils into a round balloon. Then blow up the balloon so it's about 16 inches around and knot it. For the finishing touch, tie several long ribbon streamers around the neck.