Formal:
* Dydd da i chi! (dee-th da ee khee) - This is the most formal and literal translation, meaning "Good day to you!"
* Cawsoch ddydd da! (kow-sokh thuhth da) - This means "Have a good day!"
Informal:
* Dydd da! (dee-th da) - This is a more casual "Good day!"
* Hwyl fawr! (hwihl vour) - This means "Goodbye" but is often used as a way of saying "Have a nice day" in informal situations.
Other options:
* Gobeithio y byddwch chi'n cael diwrnod da. (go-bay-thee-oh uh buh-thwch khee'n kayl dyuhr-nohd da) - This means "Hope you have a good day."
* Cewch ddydd da. (kew-kh thuhth da) - This means "Have a good day." and is a bit more informal than "Cawsoch ddydd da!"
Ultimately, the best way to say "have a nice day" in Welsh depends on the specific situation and your relationship with the person you are talking to.