Make sure to let your room host know, ask him if he can do something special. A few years ago my sons gf had her birthday while we were on the Fantasy. The room host made a lovely birthday cake out of towels, it was amazing and a really nice no cost surprise.
also if your servers are aware they will bring out a dessert and sing happy birthday to her. If she’s not a dessert person they will bring out a plate with happy birthday written on it in chocolate. Get the birthday button that a prev poster mentioned and have her wear it. She will get tons of happy birthdays from cm,s and you never know, she could get a bit of pixie dust.