How can I make this project into an experiment where students need to test/hypothesis/conclusion?

You could have students make their own cars (probably in groups) and maybe change certain variables each time. You could have them change wheel size, balloon size, terrain, anything. They could make hypotheses beforehand about which variable would affect car speed or distance the most, and write a conclusion where they say which ones had the greatest effect and maybe how the whole experiment shows Newton's 3rd.