watch

Good activity to prepare scouts for a presentation since everyone winds up sitting quietly. 
Some reward for the 'winner' might be first in line for snacks or first up for next game.

Everyone stands and closes their eyes.
On 'GO' signal from the leader with a watch, everyone tries to estimate when a minute (60 seconds) has elapsed while keeping their eyes closed. When you think it has been a minute, you sit down.

The scout that sits closest to a minute wins.

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An alternative is to have everyone do an activity for exactly one minute - jog in place, do jumping jacks, hop from foot to foot, turn in circles, ...