Good birthday card messages range from the short and sweet, such as "Happy birthday" followed by the person's name, to the long and heartfelt, including fond memories or expressions of gratitude. It is a good idea to personalize the message to the recipient, regardless of the message length, even if this just means adding their name.
If the birthday card recipient is turning a round number, such as 50 or 60, or a milestone age, such as 18 or 21, it is traditional to mention this in a positive or humorous way.
Sometimes, birthday cards are difficult to find the words for due to the circumstances of the recipient. If they are experiencing difficulties in life, for instance, it might seem trite to wish them a "happy birthday" in the conventional sense. However, there are ways to put a positive spin on the occasion, such as telling them to celebrate their persistence or courage, wishing them a good next year or telling them how much they deserve to have some fun.
An alternative or addition to writing an original message is to use the words of others. At the website of the Academy of American Poets, users can browse poems by occasion. Among their selection of poems suitable for birthdays are Carl Sandburg's 1918 "Still Life" and Joy Ladin's 2015 "Time Passes."