Shoot the Chutes Racecourse
You will need: a wall or other flat, vertical surface; tubes, boxes, tubs, cups, etc.; scissors; tape; and marbles.
Cut holes in the tubes, etc., where necessary to let a marble through.
Get permission from your parents, then tape the tubes, etc., to the wall in descending zigzags (see illustration for an example) so that a marble will roll from one to another all the way to the bottom.
Start two marbles down the course side by side and see which finishes first.