You can say:
- Many happy returns of the day!
- Many more happy returns!
- I wish you a wonderful birthday!
- Have a great one!
- Have a good one!
- May your birthday be filled with laughter!
- May you have a fantastic day and many more to come!
- I hope you have a wonderful birthday.
- I wish you all the best on your special day.
- I hope you have a fantastic day and a fantastic year to come.
- Wishing you a day that is as special as you are.
- Have a fabulous birthday!
- Wishing you many more candles to blow.