This Activity is meant for Boy Scouts, Webelos scouts.
Decide for yourself if it is appropriate for your younger scouts or not.

8-10 1/2inch dowels, 3 feet long
16 to 22 scouts

Only one scout is climbing at a time, so there may be too much 'watching' time in a larger group. Takes about 8-10 minutes for 20 people to complete.

Pair up scouts and give each pair a dowel to hold.
Line up pairs in a line close together so dowels form the rungs of a horizontal ladder.
One of the scouts of the first pair in line climbs onto the ladder and climbs it to the end. Meanwhile, the other scout walks to the end of the ladder with the dowel and waits.
When the climbing scout arrives, he joins back up with his partner to make another rung.
The next pair of scouts in line do the same thing once the climbing scout is far enough along the ladder.
When all pairs have gone through, the other scout in each pair gets to climb.