There are so many ways this question could be answered!
1. Make sure you have your retirement income lined up. Pensions? Investments? Social Security? Whatever, as long as you have enough income. Don't forget health care.
2. Make a list of how to contact all of the above income sources, in case there is a problem or for a change of address in case you move. I never had to, but it is nice to know how to contact them if necessary.
3. Write a wonderful and touching "goodbye e-mail" to send to everyone on your contact list the morning you retire. Thank them all and so on. Make 'em cry big tears. I didn't include any non-work e-mail address in it although the more extroverted here might want to do so.
4. Clothes. Keep an eye out for sales on the type of casual clothing you will want to be wearing during retirement. If you find any and buy them while you are still working, you won't have to spend first year retirement money on them.
5. Destinations. Personally I don't like to travel, but if you do then why not think about places you might like to visit? That should make the time fly.
6. Things to do. Make a list of a dozen or two things you really want to do in retirement. You will probably file this away and never use it, but if you need it then it will be there for you. Mine is still sitting on my computer desktop, untouched. I may NEVER learn Spanish or take up piano again, at this rate.
7. More seriously, try to make a smooth transition at work, for whoever takes on your responsibilities. I wrote a 150 page manual for my supervisor and replacement only, on how to do the most enigmatic part of my job. Don't leave tons of useless paperwork for others to have to sort through - - throw out the papers that are trash, and distribute the rest to the people that need it. The same goes for e-mails.
8. Give away the GOODIES in your office (that belong to the organization) to those who deserve them. The ergonomic chair, that rare 3-hole punch that you keep hidden, the binders full of hard-to-find reports, the bookcase. The vultures are rabid for this kind of thing, and wouldn't you rather give them to the deserving?