I think human paintings or writings on the cave wall. This will be the best thing. It is believed that ancient humans inscribe paintings on walls to pass down information to offsprings.
There was no internet, notepad, libraries then, so the best way they could store this information is writing on walls. Not just any wall that could be demolished anytime.
The cave is the perfect place. Finding a cave painting or writing will mean a lot as it can change the world. It will let us have more knowledge about the past generations