Formal:
* Kia pai tō rā (Formal, literally translates to "May your day be good")
Informal:
* Haere rā (Informal, literally translates to "Go [and have a good day]")
* Kia ora rā (Informal, means "Good day")
You can also add a specific wish, such as:
* Kia pai tō rā i tēnei rā (Have a good day today)
* Kia ngāwari tō rā (May your day be easy)
* Kia whai hua tō rā (May your day be fruitful)
The best choice will depend on the specific situation and your relationship with the person.