nice retirement greeting?

Its hard to say something meaningful when you have been co-workers for such a short time. You could congratulate him on a good career and wish him well in his retirement ventures, thats about all I can think of that isn't personal. I'm sure they will appreciate the thought regardless of what you write in it.

If you only had a short working relationship and don't know them on a personal basis I don't see anything wrong with a simple "best wishes on your retirement" type statement. Anything else would seem kinda phony to me. Write what you feel. If you don't have any feelings about it one way or the other, then stick to the standard deal "best wishes" type statement.

In some other countries, it's considered a really good thing, and is celebrated as entering their "second childhood". That's how I see it. I always say, "Wow, you're getting ready to enter your 2nd childhood, have fun!" At my age I get to say this alot.

How about "Congratulations on your retirement. I'm jealous and I hope I can stand in your shoes someday!" ?

Corny, I know, but being corny and sincere are what its all about.