Two really series of ones:
In Texas you have all these old cemetaries everywhere, along old trails,
former farms,etc and those each tend to get a geocache. Then there are some
challenges to get "cemetary caches" for a certain number of counties or in a
certain area.
In London, England there use to be a series of caches that were the various
plague pits; the plague pits were the places they disposed of the bodies
during the black plague since they were not able to properly bury all the
dead. Some are really neat because the locals have kept up memorials other
are just under current parks and garden.
The ones I kind of look out for when traveling are the old memorials on the
side of the road because of auto accidents. People have put out stones and
metal markers and they have to be far off the side of the road, then over
time the places get overgrown, the roads get moved and contruction workers
move or work them into the ground, etc. They are just forgotten memorials.