Those secret reeducation camps. If you find one, lots of pictures, go home, and write a bestseller. They are scatted along smaller trails off of the main roads. The Ho Chi Minh Trail. Unexploded mines and bombs.
Villagers often use war scrap metal to line their gardens.
If you hike up into the hills you may run across a downed chopper or rusting tank that was too inaccessible to haul away for scrap. Those perfectly round fishponds are bomb craters. They're beautiful, and they're all over the place. Dalat has great strawberry jam. Dalat is built on a steep hillside.