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.
Oh BTW I am currently waiting to complete a “challenge” (its an achievement) for a special game, with a special achievement. All I have to do is, not to play the game. No seriously, “The Stanley Parable” has a famous achievement, that you get if you don’t launch the game for 5 years. The fun story is, I purchased the game just to get this achievement. Really. I purchased it and waited 5 years, then installed it and run it.
But wait, why don’t I get the achievement? After an investigation I came to realize that the game has to run at least once, so the timer starts counting. Well, since then I played the game and wait another 5 years. I almost reached the fifth year. So to complete everything (which I did not honestly) you would need to do not to play the game. Is it worth it? I say absolutely!
The Stanley Parable is a meta game - a game about playing and making games. And there you are having fun not playing a game…