Hǎohao wán!

好好玩!

Have a good time! 

Notes:

好好(hǎohao): do something properly

玩(wán): to play

Example:

A: 明天我们去度假。

    Mínɡtiān wǒmen qù dùjià.

    We will be on vocation tomorrow.

B: 好好玩。

    Hǎohao wán!

    Have a good time!

Level: