Question: Normally, most people wish someone "Ad meah ve'esrim" ([May you live] to a hundred and twenty). What do you say on a person's hundred and twentieth birthday.

I have seen the answers about why 120 connected to Moshe Rabbeinu and that this is considered the maximum age (even though Aharon lived to 123, and Miriam to (about) 133. I have also seen the references to Parshas Noach, even though there are meforshim who say the reference is to the number of years until the mabul and not the maximum age that humanity would start decreasing towards.

Note that I first heard this as a joke, but serious answers are being sought here.

The original answer that I heard was for the joke and not serious ("Have a nice day") so it would not really be appropriate for this question.

Answer: 
According to Rabbi Shurkin in his *Meged Givas Olam*, some great rabbis he studied under were against the idea that one can only live to one hundred and twenty years.

He cited an incident where Reb Moshe Feinstein wouldn't fill out a certain life insurance policy when he realized it would only cover him till one hundred and twenty years.

Also, if I may add, our tradition tells us Chutzpis Hameturgaman was murdered the day before his one hundred and thirtieth birthday, so the one twenty idea seems to have no real basis.

So, in closing, wish them many more happy years of life.
Given the plethora of blessings that one may give on someone's birthday, the answers to this question are innumerable.
So the answer is - any blessing except for Ad 120.