Formal:
* 祝您今天愉快 (zhù nín jīntiān yúkuài): This is the most formal way to say it, literally meaning "wish you a pleasant day."
* 祝您一天順利 (zhù nín yītiān shùnlì): This means "wish you a smooth day."
Informal:
* 祝你今天愉快 (zhù nǐ jīntiān yúkuài): This is a more casual way to say it, used with friends or family.
* 今天過得開心 (jīntiān guò de kāixīn): This means "have a happy day."
* 玩得開心 (wán de kāixīn): This means "have fun" and is used when someone is going out or doing something enjoyable.
You can also simply say:
* 再见 (zàijiàn): This means "goodbye" but is often used as a way to wish someone a good day.
The best way to choose which phrase to use depends on the situation and your relationship with the person you are talking to.