Formal:
* Tenha um bom dia! (This is the most common and polite way to say it)
Informal:
* Tenha um ótimo dia! (This is a more enthusiastic way to say it)
* Bom dia! (This literally means "good day" but can be used as a friendly way to wish someone a nice day)
* Boa tarde! (This means "good afternoon" and can also be used as a friendly greeting)
Other Options:
* Que o seu dia seja bom! (This translates to "May your day be good!")
* Aproveite o dia! (This means "Enjoy your day!")
The best way to say it depends on the specific situation and your relationship with the person you're speaking to.