I would go back to 2010 and tell myself, "buy Bitcoin, sell on Dec 17, 2017, buy after it drops and sell again on April 14, 2021. If you still need money after that, reevaluate your life".
Same. Or maybe winning lottery numbers. Telling myself to invest in tech companies like Apple or Google in the late 90s won't work because I won't have the money or means to do it.
It probably wouldn't work, but I would warn Martin Luther King's event organizers or security detail.
I'm old enough that I could have lived in a world where Martin Luthor King Jr was still an active activist in my lifetime. It's heartbreaking to think about what his loss meant not just symbolically, but in terms of how much better the entire world could have been with almost an extra half century of his influence.