[vpmedia]

well, thanks for asking:

- returning smoking bomber/recon plane over the Channel coming from the opposite direction, maybe escorted by few fighters

- seeing a returning smoking fighter coming home, while waiting for takeoff

- being jumped by a pair of fighters at unexpected locations where you feel secure otherwise

- seeing few aircraft battle over your field while takeoff, your side is in advantage

- seeing convoy of ships being attacked by stukas or coast artillery

- german speedboat formation engaging ships

- smoking/burning damaged ship, leaking oil maybe, sailors in lifeboats

- floatplanes operating, rescue operation in progress, pilots in water

- huge smoke columns in the distance, distant artillery fire

- trafic on the roads being attacked

- fishing boats operating and being attacked

- barrage balloon being hit by pair of fighters

- sudden starfing attacks out of nowhere

- enemy plane hitting ground while escaping in panic at low alt (for rookie pilots)

- enemy aircraft scrambling from airfields when attacked

- groups of horses/cows/sheep near farms/rural areas

You need very visible/oblivious things which capture the players attention
imho smaller details remain ofter unrecognized by the user.