Anything that enhances the setting/mood.
At Border Wars 2, I could hear the loudspeakers of a nearby racetrack, and I thought: how cool would it be to use loudspeakers to establish certain settings and moods? For example, it would be cool to have a base that was blasting foreign language propaganda over its loudspeakers.
Another game at the LZ, there was a burning car. A burning car is ALWAYS cool, but it really set the mood / added to the realism to have billowing black smoke in my field of vision.
KWA KM4A1 - CA MP5SD3 - KWA KMP9 - WE Wilson Combat CQB Elite