Super hot, all-day long birthday sex. LOL, no, not yet any way. Maybe a good book, something he has always wanted, or a rare, hard-to-find copy of a favorite author's work. Damon seems to enjoy reading, so somethng along those lines Elena could give. Maybe, a very nice shirt or jacket-black of course.
I think Damon will try and get Stefan back for Elena's birthday. Elena would probably love that gift and it would show how much Damon loves Elena and how selfless he is when it comes to her.