What trivial tasks are essential post closing?

Hey,

I'm (hopefully) a few weeks away from closing my deal. And I started thinking about what would happen after the transaction closed. Not 3 months later, but the day after. I suppose you have a lot of administrative tasks to take care of, like managing the bank account, signatures, access to company systems, etc. I don't see that in any books.

What's your list of the most important little things to do after closing?