Either one or both works.
Mine is completing the Pokedex in the original Pokemon games. All you get is just Professor Oak giving you a wink and a small few second cut scene. And a congratulations text. Imagine spending all of your time then, getting all 151 and even 252 pokemon just for that? Yeah no thanks, I never completed the pokedex.
Going the Joja-Route in Stardew Valley. I say this mainly because, it is what you make of it. You forfeit being able to complete the Community Center by earning things, when you sign your soul away to Joja. What I would’ve liked is seeing Pierre go out of business completely. I just think that would add a route of depth in the game where you have to make ends meet through Joja because Pierre is permanently closed.
But, that doesn’t happen, he’ll still be in business despite his depression about Joja running things. Kindof ruins the whole concept of doing it for the achievements even.
After someone on Lemmy recommended Dwarf Eats Mountain (it’s okay), I checked out the idle game genre for the first time.
On one extreme, Magic Archery was completed in under an hour and all seven achievements were earned during normal gameplay.
But most other idle games, ho boy. They tend to have several hundred achievements, many of which would take literal weeks if not months to achieve, and often require resetting the game back to the start dozens of times due to prestige mechanics that are necessary for late-game progression.