The words to express to an employer who is retiring will depend largely on the employee's working relationship with the boss, but a congratulations and well wishes for the future are commonly expressed. Congratulations can be expressed either in person, in writing or in the form of email. Again, this depends on many factors, including how closely a person has worked with their boss.
If there is a card circulating within the office, it is completely acceptable to say only a few words of well-wishes. Phrases such as "I wish you the best in the next chapter of your life," and "your presence will be missed, congratulations!" are short and express sincerity. If the employee has purchased his or her own card, more sentiment will need to be expressed. This is where the employee can thank the boss for all that they have done with the company.
Sending an email of congratulations is less personal but acceptable. Wording can be a little less formal, and can use phrases like "congrats" and "thanks." Additionally, an employee can wish an employer well face-to-face. This should be done completely in the employee's own words without catch phrases. Lack of sincerity will easily show if a person is face-to-face. Generally speaking, signing a card that travels through the office or writing a quick email is perfectly acceptable.