Leave a piece of life advice.
Doesn't matter if it's specific or more general. Doesn't really matter if it's your original though or advice you've picked up along the way from someone else.
If you were going to give someone you don't know some advice, what would it be? Something that could be beneficial to a stranger out there.
"So toss away stuff you don't need in the end, but keep what's important and know who's your friend" - Phish
Lyrics but I feel it's good life advice.