Some crazy idea people will laugh at

[Twoflower]

Don't be lazy and just bomb things, build elaborate grief traps that greyshirts only wish they understood.

and while you're designing the elaborate trap the captain comes and lasers you through the window

sell snake oil to the crew in the form of the deadliest combination of chemicals that you can find. Portioned out so that it's so weird even the doctors won't understand the cause.

i don't think many people are going to buy anything from a chemist on Grief Day

Determine how to annoy people's pda's in the most efficient way possible using mechanics. Discover if it's possible to hijack every message people send and replace it with the string "Butts". Including the alerts.

again, while you're designing it you'll be lasered

Give AI a law designed to send the crew on a scavenger hunt looking for a traitor that doesn't exist.

who's going to want to hunt for a traitor when everyone's a traitor

everything you said (except for the last one) is stuff that people already do regularly, so i don't see the need to have a whole day for it