Formal:
* Miłego dnia! (mee-leh-goh d-nya) - This is the most common and polite way to say it.
* Udanego dnia! (oo-da-neh-goh d-nya) - This means "successful day" and is slightly more formal than "Miłego dnia."
Informal:
* Miłego! (mee-leh-goh) - This is a shortened version of "Miłego dnia" and is more casual.
* Dobrze Ci się baw! (doh-breh tsee sheh bahf) - This literally means "Have fun," and is a good option for friends or family.
* Dobrego dnia! (doh-breh-goh d-nya) - This is a more general and friendly way to say it.
You can also add a specific wish to your greeting:
* Miłego dnia w pracy! (mee-leh-goh d-nya v pra-tsee) - Have a nice day at work!
* Miłego dnia w szkole! (mee-leh-goh d-nya v sh-koh-leh) - Have a nice day at school!
The best way to choose the right phrase is to consider your relationship with the person you're talking to.