“Happy birthday, hope it’s a great one” is what everyone writes. One shared memory and one thing you appreciate about the person is enough to make a card worth keeping.
Who they are to you.
Sister, oldest friend, mentor, grandfather. The relationship sets the voice.
A memory from this year.
Something you did together, or something they did that stayed with you.
What you appreciate.
A quality you would miss if they were different: their humor, their steadiness, their terrible puns.
A wish for the year ahead.
Tie it to something real in their life, such as a new job, a move, or a trip they are planning.