1) Chef Mickey's isn't too special on birthdays.
2) You usually get a cupcake, and sometimes a card signed by characters.
3) Once in a while, a signed placemat.
4) During the meal, they ask for birthdays, and they sing a song for everyone.
NOTE: Birthdays, anniversaries, etc are special to people, but they happen hundreds of times per day at WDW. Thus, they can't spend all that much time on individual events. In most cases you get some kind of recognition, but not a lot of special items.