There is a second Ribbon to be found in the game, besides the one in the Temple of the Ancients (I didn't even know about it until very recently, have never got it myself yet, only read about it so far). It's somewhere in Gaea's Cliff, inside the caverns.
And there's a materia in the final dungeon that is behind a waterfall or something that is only very barely visible in the PSX version (it's a lot more apparent in the PC version).. I can't remember what it is, as I've never got it myself.. can only remember reading about it and seeing a screenshot of the location.
I think that's the only 2 things I've ever heard or read about that I have never got in the game personally. But I'll be sure to get them in my next time through, whenever that will be.