Formal:
* يوم سعيد (Yawm Sa'eed): This literally means "Happy Day" and is a very common and polite way to say goodbye.
Informal:
* يوم جميل (Yawm Jameel): This means "Beautiful Day" and is a more casual way to say goodbye.
* أنت بخير (Anta Bikhayr): This means "You are well" and is a very friendly and informal way to say goodbye.
More specific:
* أسعد الله أوقاتك (As'ad Allahu Awqatuk): This means "May God bless your time" and is a very polite and religious way to say goodbye.
It's also important to consider the gender of the person you are talking to. In Arabic, the verb "to have" changes depending on the gender of the subject. So, if you are talking to a woman, you would say "يوم سعيد (Yawm Sa'eed)" or "يوم جميل (Yawm Jameel)". However, if you are talking to a man, you would say "يوم سعيد (Yawm Sa'eed)" or "يوم جميل (Yawm Jameel)".
Ultimately, the best way to say "have a nice day" in Arabic depends on your personal preference and the specific situation.