ABOVE - Here are some little men to knock over. Do you think that you should get more points for getting the marble through their legs or for knocking them down?

BELOW - A different game. Roll your marble through one on the 'doorways'. Do you think they should be the same size?
What happens when you hit the wall?

Here is something else to try.

Pieces of wood glued to a sloping board. Set the marbles rolling at the top. How long does it take them to reach the bottom?