What do you write down when planning a session, an adventure, or a campaign?

When planning a session, I like to jot down a few key events that will need rolls that are likely to come up. In call of cthulhu or delta green this will be sanity check roll events as I want to make sure these actually happen!

But regardless of the system, for one shots I definitely plan 'beats' as it's annoying to run out of time to conclude whatever's going on.

I try to keep NPC details light so they're adaptable. The important stuff is stats and skills, the rest you can improvise