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！