This is quite a good project, the easiest one to do is an air propelled vehicle, made using an empty plastic drinks bottle, a balloon, and some wheels from a toy car.
Insert the balloon inside the drinks bottle but keep hold of the end the you fill with air, you will need to keep this part hanging out of the hole.
You will then need to fix your wheels to your bottle, and make sure they turn freely to maximise the distance your newton scooter will travel.
once you have this all set, drill a small hole into the cap on the drinks bottle and pull the open end of the balloon through this and then screw the cap back onto the bottle.
Once this is all done, decorate your scooter.
All that is then left to do is inflate your balloon and then release your scooter allowing the air to escape and propel your newton scooter forwards.
Tip: Make sure the hole in the cap is around 5-7mm to increase pressure of the air which will be released from the balloon.
Hope this helps!