On my birthday not so long ago, I logged into SO, and was pleasantly suprised to find an unexpected rep bonus: Someone had discovered a very old question of mine, upvoted it, and added an answer, so that it appeared on the front page where it gathered a few other upvotes. A nice birthday present.
I recently came across a few questions about birthday presents on meta, and it occurs to me that one way SO could give a birthday present without revealing users birthdays: Have the Community Bot automatically bump your highest-scoring question or answer to the top of the heap on your birthday, giving you a chance for added rep as a birthday gift.
Since the bumping algorithm is shrouded in mystery, there would be no way for evil-doers to discern your exact birthdate by scanning the front page. And it addresses the objection that a direct rep reward doesn't properly reflect community trust, since only worthwile posts will garner more votes - the bumping just increases the chance of your wisdom being noticed.