Formal:
* Miłego dnia! (mee-leh-goh d-nya) - This is the most common and neutral way to say "have a nice day" in Polish.
* Życzę miłego dnia! (zhi-che mi-leh-goh d-nya) - This translates to "I wish you a nice day" and is slightly more polite than "Miłego dnia!".
Informal:
* Miłego! (mee-leh-goh) - This is a short and casual way to say "have a nice day".
* Dobrego dnia! (doh-breh-goh d-nya) - This translates to "Have a good day" and is a bit less common than "Miłego dnia!".
Other options:
* Miłego popołudnia! (mee-leh-goh poh-poh-wood-nya) - This means "Have a nice afternoon".
* Miłego wieczoru! (mee-leh-goh vee-choh-roo) - This means "Have a nice evening".
The best choice for you will depend on the situation and your relationship with the person you're talking to.