Oil barrels, rope, ammo crates...you've tried them all.
What are you trying to acheive? You looking to identify those who can inspire, improvise and motivate their team toward a common goal.
How about some dislocation of expectation? Set them a non-physical task that has some mental problem rather than a purely "who can shout the loudest". Use some examples from recent conflicts, such as a hostage rescue, an injured man down scenario, or even to plot a WWII escape from a stronghold. See who can propose their ideas in a coherent manner, and inspire others to try it.