Challenge the Cubs or Scouts to build a structure to hold a long length of guttering that a marble can roll down. Get sixes or patrols to compete against each other by seeing which one can make the run that takes the marble the longest to reach the floor.
Pioneering staves (6 or 8 per team, either all the same length or the same mixture of lengths)
Guttering (1 per team, all the same length)
It might be a good idea (especially for Cubs) to build a tripod beforehand as an example
Give each team the resources they need and the following instructions:
You need to build something that will hold the guttering away from the ground.
The guttering must have a slope large enough to make a marble run from one end to the other.
The winning marble run will be the one that takes the longest for the marble to hit the ground (or the end of the guttering if not open-ended)