I think - beside the DLC - the two things that are most hidden and most likely to be missed by a natural first playthrough are The Great Hollow and the Painted World of Ariamis, so feel free to look those up if you haven’t found them after you’re satisfied poking around on your own. But yeah as you’ve noticed the game is absolutely chock full of secrets.

Do make sure you get to the DLC too, it’s got some great content, as well as the most fun bosses in DS1 - with one of them being an absolute icon. Still won’t be very challenging compared to Elden Ring, but a step up from the base game.