I love a mission that surprises me and rewards free thinking and initiative. The downside is some players will never see the payoff.
Subtle hints are cool, like in the briefing or intro, you can add "Additionally, use your own initiative to disrupt the enemy." The disruptions can take the form of destroying communications, destroying a fuel station, mining a road (all of the preceding might be good for delaying enemy reinforcement for example).
In game hints are good too, like overhearing two enemy guards talk about inbound armor that is low on fuel, or "the radios are jammed, so land-line phones are our only communication", etc.