Decorated paper made into a tube makes a fun indoor paper rocket. And easy! Tape up one end and slip a straw in the other end. Blow in the straw to launch the rocket. Hubs also had the idea of adding a fin to enhance stability but we didn't get that far. Next time.
What we did do is set up a target field in our hallway which turned out to be loads of fun.
Check out the full tutorial on making paper rockets at Curious Kangaroos.