I would use the rocket launching activity with my 2nd class students.
My stimulus would be a video of an actual rocket launching. I would ask the students to tell me everything they know about what is happening in this video. We would then discuss rocket launches, where the rocket might be going, what its purpose might be. I would tell the students that today we are going to investigate how a rocket launches.
I would show them the Marvin and Milo cartoon of the balloon rocket. I would leave that displayed on the board. I would divide the class into mixed ability groups and provide them with all the equipment they need to carry out the experiment. I would circulate between the groups, offering support and help when needed. Once the students have successfully launched their rocket, I would encourage them to think of anyway they might be able to change one thing in the experiment to see if the results would be different, e.g. using a different sized balloon, blowing the existing balloon up more/less, etc.
Once all groups are finished, we would discuss what they have learned. I would ask them if they can guess from their findings in this experiment what force might be behind the launching of a rocket into space. We would discuss how, in a real rocket launch, gas released from the ignited fuel pushes down hard against the ground pushing the rocket up into the sky.