You can probably answer this one for yourself. You know what he likes to do, what kinds of movies he likes, what music he listens to. Buy him something related to his hobby, a special edition DVD of a movie by his favorite director, or a rare CD by his favorite band. Get a book autographed by his favorite author.

Whatever he finds sexy, play it up for his special day. If he loves your hair worn a certain way, in pigtails, for example, tie it up in ribbons of his favorite color. If he loves your feet, paint your nails in his favorite color. If he has told you he likes certain makeup you wear, put it on for him and make sure it looks extra nice. Wear the perfume he likes best on you. Talk to him and tease him in the sexy way he loves. A sexy gift of yourself make a great birthday gift.

Plan a day or a date to do something he says he has been wanting to do. Take him to a museum exhibition he has been wanting to see. Has he never seen his favorite band live? Surprise him with tickets. Take him to the new movie he said he wants to see.

This might only work if you actually know your boyfriend really well. You might have a sense that he is really interested in trying something different, but is afraid to say it for fear of being embarrassed. You can be the one to really introduce him to a new interest. Buy him a book on the subject. Or take him to an event focused on his suspected interest. Remember, guys don't want to look foolish, so you'll have be discreet. If he asks him why you bought it for him, or why you brought him to the new place, tell him you had a feeling he'd like it.