I’ve been trying to remember the details of my eldest‘s study. She did this six years ago.
I did buy her a couple of 1metre steel rulers (set up on our flat dinner table), so that the velocity of the objects, before & after the collision, could be calculated from the distances each object covered in the time segments, measured by a stopwatch app on a large iPad, captured on video, by a second iPad. The edge of the ruler helped to keep the objects rolling in a straight line, but there is some loss of energy from friction (which is present between the object and the table surface anyway, so should be estimated & discussed).
She used objects of different masses to show how conservation of momentum caused smaller objects to travel at higher velocity after the collision. She tried different sized drink cans - 375mL, 250mL, 200 mL and empty cans (their cylindrical shape make them relatively easy to roll) and different sized marbles. Experimental designs often involve trial & error.
Good luck.