One funny birthday wish is "Smile while you still have teeth!" Another is "I wish you peace, love, wisdom, and everything else that doesn't cost money." Jokes about birthdays often reference the cost of a gift or the age of the person celebrating.
Other messages to write on birthday cards include "Birthdays are good for you--studies have shown that people who have more of them live longer" and "Better over the hill than under it." Another humorous message is "Birthdays are nature's way of telling us to eat more cake" or "Happy seventh 30th birthday" for a person who fibs about his age.