Loot caves. Even if your reward was just cash and an achievement, a cave with loot would require you to design all: 1) A satelite to find a cave 2) A rocket with enough deltaV to get you there and back 3) A rover capable of exploring the cave 4) Whatever you find, it might force you to make ANOTHER rover to finish extracting that loot.
You know what is KSP missing?
Loot caves.
Even if your reward was just cash and an achievement, a cave with loot would require you to design all:
1) A satelite to find a cave
2) A rocket with enough deltaV to get you there and back
3) A rover capable of exploring the cave
4) Whatever you find, it might force you to make ANOTHER rover to finish extracting that loot.