Hmmm, tough one. I'd have a few but three cases that immediately come to mind are:
1) Completing Star Wars: Shadows of the Empire (N64) on the Jedi difficulty AND finding all the Rebels collectibles ("Challenge Points" I believe they were called).
2) Again on the N64, with GoldenEye 007, completing the game something like 3 times I think, or maybe 4 in order to then unlock ALL the cheats by completing the corresponding timed 'challenges'. I ended up finishing all of them but it wasn't without extreme frustration, yelling and skipping breakfast / diner for about a full week. Then a few years later learning in a magazine that push-button codes existed, it pissed me off a little bit as I was remembering how tough it was (at least for me) to do all of that properly but fortunately it had been a few years so I didn't really care anymore.
3) On the Dreamcast version, finding ALL the 'Cadeaux' collectibles in Shadow Man (I loved that game so much, one of my all time favorite third person action / pseudo horror adventure). Something that to this day I've never checked the facts on is that I remember collecting all of them since when one did so, it then maxed out something in return for the player, on one of the 'meters' for the character (life bar or something, can't recall). However, by the end of the game I remember that I kept finding a few more, not many, maybe something like 10 or so 'loose / extra' Cadeaux that perhaps weren't meant to be found or were there to serve as a backup in certain maps in case players would skip a few during their playthrough and would have some room for mistakes and still end up finding "all of them". I think that the final official (needed) number for maxing out whatever it did was about 500, but I always ended up finding about 510 from what I can recall (or maybe a bit less, something like 505, 506 or so). It wasn't as extreme in comparison as something like finding 999 seeds in Breath of the Wild, but at the time finding those Cadeaux was enough of an accomplishment for me to still remember about it today. I think I completed that game a good 4 or 5 times so I eventually just memorized every imaginable nooks and crannies.
I'm sure there's others I'm not thinking of right now but these three above will stay with me forever.