Make some shapes out of paper that can fly, but the challenge won't be to make rockets or airplanes.
Show them a demo, then let them take it apart and figure out how it works.
After that, they can try making their own gliders. Ask them to make either smaller or larger than the demo and let them figure out the number of pieces of paper they would require.
Questions to be asked
Why do you think a circular shape flies well?
Can you figure out a way to make it without using any tape or glue?