Absolutely nothing.

We’re living in the world, where there are only three possible outcomes to a cataclysm:

• whoever survives it, will regret it
• the cataclysm won’t be that dire and the aftermath will be either temporary, or it will bring limitations we’re going to accept and continue to live like before
• we die

So, I don’t plan for the aftermath. Whatever will be, will be.

That being said: worldbuilding and inventing characters with their relevant stories is an interesting mental exercise that requires nothing but mind, and may be shared with other survivors. Same goes with certain narrative role-playing games.

I don‘t plan for that explicitly, but I would go for analog games and books.

probably work on survival tasks?

I guess it would be good to try to figure out low power computing and then you could just work on all kinds of things on a computer: reading, writing, coding, etc. (mobile phones that you hook a keyboard and mouse up to would be pretty low power for general computing I’d guess)