Formal:
* ขอให้มีความสุขในวันนี้ (khor hai mee kwahm sook nai wan nee): This is the most formal way to say it, literally meaning "May you have happiness in this day."
* ขอให้มีความสุขตลอดวัน (khor hai mee kwahm sook dtlaat wan): Similar to the first option, but emphasizing the whole day.
Informal:
* มีความสุขนะ (mee kwahm sook na): This is a more casual way to say it, meaning "Have happiness."
* สวัสดี (sa-wat-dee): This is a general greeting, but it can also be used to mean "Goodbye" and "Have a nice day."
Other options:
* ขอให้วันนี้เป็นวันดีๆ (khor hai wan nee pen wan dee dee): This literally means "May this day be a good day."
* มีความสุขกับวันของคุณ (mee kwahm sook gap wan khong khun): This means "Have a happy day with your day."
The best way to choose the right phrase depends on the situation and your relationship with the person you are speaking to.