Many happy returns of the day.

年年有今日，岁岁有今朝。／福如东海，寿比南山。

Happy 24th birthday!

24岁生日快乐！

Wishing you a wonderful birthday.

祝你过一个愉快的生日。

All the best!

祝你一切都好！

Have a great/smashing one!

祝你（生日）玩得开心！