I did a lot of stupid tedious stuff because it was a prescribed thing to do in the game or an achievement, but I guess the one that stands out that I did of my own volition was in the original Deus Ex. At the end of the intro mission you're faced off with an NPC named "NSF Terrorist Leader". I knocked him out and carried his body through the whole game, because bodies would persist through loading transitions -- even ones that spanned the globe. I just did it because, I dunno, I wanted to see how far I could take him, and I managed to get all the way through lugging his unconscious body slung over my shoulder. I just loved that it was a thing that I could do in the game, so since then I've tried finding other games where such a thing is possible, but I have yet to find another.