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!