A message for a co-worker's retirement card can be simple, such as, "It's been a pleasure working with you! Best wishes on your retirement!" The message can also be sentimental, such as, "You were always a great co-worker, but you were an even better friend."
Some messages for a co-worker's retirement card can be comical. Some good examples are: "God retired after only six days. You took much longer!", "After years of double shifts and horrible coffee, you are finally retiring! We can't do anything about the horrible coffee, but you'll have more flexible hours," and "Since you no longer have any deadlines to meet or bosses to please, you'll probably need to take me with you to keep you from getting bored!"
Some more formal suggestions for wording a message include, "You'll be remembered for your many invaluable accomplishments," "Thank you for your many years of hard work and dedication," and "Wishing you health, happiness, and relaxation in your retirement."
If you are close with the co-worker that is retiring, you can tailor your message to express your wishes. Write, "I'm going to miss our morning chats in the break room" or "It won't be the same without your funny memos in my inbox."