Informal:
* 周末玩得开心! (zhōumò wán de kāixīn!): This is the most common and straightforward way to say it. It literally translates to "Have fun this weekend!"
* 周末愉快! (zhōumò yúkuài!): This is a more polite way to say it and literally means "Happy weekend!"
Formal:
* 祝您周末愉快! (zhù nín zhōumò yúkuài!): This is a very polite way to say it and literally means "I wish you a happy weekend!"
You can also add "and have a good rest" to any of these phrases:
* 周末玩得开心，好好休息! (zhōumò wán de kāixīn, hǎohǎo xiūxī!): Have fun this weekend and take good rest!
* 周末愉快，好好休息! (zhōumò yúkuài, hǎohǎo xiūxī!): Happy weekend and take good rest!
* 祝您周末愉快，好好休息! (zhù nín zhōumò yúkuài, hǎohǎo xiūxī!): I wish you a happy weekend and take good rest!
Ultimately, the best way to say it depends on the context and your relationship with the person you are speaking to.