100%ing would require me to look into guides and it's gonna be a long time, if ever, until I do that for Skyrim. I'm just gonna go in and enjoy it as much as I can (not saying 100%ing isn't enjoyable, just that doing what I'd need to do for that would lessen my enjoyment of it, at least to start with). Don't get me wrong, I'll definitely have a playthrough where I try to do as much as possible, but unearthing every single thing in a TES game, I just doubt I'll manage to do it.
As for what I 100%ed, I think GTA Vice City was one of the rare games I did. I went for trying to do as much as I could, looking up the guides, then I saw somewhere what the prize for completing 100% of the game was - being able to take 2 thugs to follow you around (which was quite rare back then).