Formal:
* 祝您今天愉快 (zhù nín jīntiān yúkuài) - This is the most formal way to say it, literally meaning "Wish you have a pleasant day today."
* 祝您一天顺利 (zhù nín yītiān shùnlì) - This means "Wish you a smooth day."
Informal:
* 今天过得开心 (jīntiān guò de kāixīn) - This translates to "Have a happy day today."
* 玩得开心 (wán de kāixīn) - This means "Have fun" and is more appropriate for friends or family.
* 祝你好运 (zhù nǐ hǎoyùn) - This means "Good luck" and is also a common way to say goodbye.
Phonetically:
* zhù nín jīntiān yúkuài (jhoo nin jin tian yu kwai)
* zhù nín yītiān shùnlì (jhoo nin yi tian shwoon lee)
* jīntiān guò de kāixīn (jin tian gwo de kai shin)
* wán de kāixīn (wan de kai shin)
* zhù nǐ hǎoyùn (jhoo nee hao yoon)
Remember that the tone of the pronunciation is important in Chinese. You can try using a Chinese pronunciation dictionary or online tool to help you with the tones.