There's a place in (I think) Corel that I never knew about, and still haven't actually been to, but I saw it in HC Bailey's Let's Play of the game. I forget the specifics, but if you go the direction you don't have to there's an area where there's at least one NPC, but I forget what their purpose is. Sorry that's vague.

As for things I discovered myself; I didn't know that there was a room in the Honey Bee Inn with the old couple talking about their son at Shinra, with Cait Sith's music playing and a little Cait Sith in their room (I think it's safe to say who their son is...) I can't remember if I saw the other room on my first playthrough or not. It also took me time to know that both of those rooms have more than one scene.

I knew about most of the things in the original post, but the Cursed Ring was the one thing I didn't know about on my first playthrough (I failed to do the thing with the kid's gil, but I knew about it.) That was a fun discovery, and I like how it wasn't what you'd expect with the whole "Locked Door" thing.

Also, next time try looking at the kids gil, but don't steal it... see if anything different happens when you see the kid after he wakes up.