What changes in different playthroughs?

I wasn't really planning on playing the game through a second time. However, when I loaded up the scene to check out the quotes ingame for this answer, I noticed that I was getting different narration than previously.

Does anything else change in the game, like the environment, items you come across, or scenes?

Solution 1:

Throughout the game there are trigger points that are in fixed positions.
Each trigger point chooses an item from a set of narration parts that are available for that specific location. So on my second playthrough I had mostly different parts of the monologue and some that were the same as the first one.

Furthermore I noticed that on the mud bank, in the cave near the blue river, I noticed the second time through there was not only a polaroid, but also a bird's nest.
Not certain if I just didn't notice it the first time or if it was an actual difference.

Solution 2:

It's completely randomized. Sometimes there is a wedding ring on the ledge in the last house before the tower. Sometimes you see a figure standing on the path ahead, always in a position where you must lose line of sight to reach it, only to find it deserted upon arrival.