One that shows he thought about her likes and dislikes and knows her.
Some gals would want concert tickets, some would rather stay in.
Some like flowers and/or chocolate, others are allergic.
Mostly, he needs to apologize for what he’s done wrong and make a sincere effort to make it right. (That’s assuming he has actually done something wrong and that she’s not just one of those gals that is angry at life in general.)