In Eastmarch near to Riften. I've just been exploring with no quests since I've done everything else pretty much, and found lots of new places to explore or clear out, but mostly quite small locations. Just when I thought I'd done the biggest places, I discover Torvald's Cave. This place is pretty huge. Not Blackreach huge, but there's so many areas and tons of falmer and falmer shadowmasters and chaurus. It goes on and on, I'm not sure if I'm near the end of it yet. Quite a challenging one to clear out if you're interested in exploring it. No idea what quest would have been associated with it.

Thieves Guild-No Stone Unturned

Ah, I see. Well No Stone Unturned is the only remaining quest in my journal. I have 15/24 stones and need 9 more to restore the crown. I didn't find the unfinished crown in the cave because it's apparently not accessible unless I activate the quest. And that can't be activated till i find the rest of the stones which frankly, I can't really be bothered with, lol.