If you have a pension, find out what it takes to get the monthly payments started or to get the lump sum. My company required something like 60 days to process a pension request, so don't wait until the last minute if you need a payment the month you retire.
Do you have enough cash saved up for living expenses when the paycheck stops? How many months/years of expenses do you want in a liquid stash like a savings account before you need to tap your investment accounts? You may want to redirect part of your final few months of paychecks to build up your cash reserves.
Decide if you need to adjust your 401k contributions based on your retirement date. If you're retiring before the end of the year, you may want to increase your contributions to hit the max by your retirement date.
If you have an HSA with employer contributions, check into its rules to see if you want to adjust your contributions.
Save a copy of your co-workers' email addresses, at least for those you may want to contact later. Establish professional connections in LinkedIn.
Prepare your farewell email in advance so on your last day all you have to do is hit 'send'. This gives you time to word it properly to not leave out anything important and to avoid saying something you may regret. Decide in advance who you will be sending it to.
While you still have access to the company's intranet, are there any HR documents you want copies of? Benefit plan descriptions that you may need to refer to after you've left?
THIS! I spent plenty of idle time during my last few months of work creating my retirement todo list. Nine months in and I haven't looked at it yet!