spontaneous

The crowd broke into spontaneous applause when the hero appeared on stage.

(Chinese) 当英雄出现在舞台上时，人群中爆发出了自发的掌声。

Sometimes the best trips are spontaneous ones where you don't plan anything in advance.

(Chinese) 有时候最好的旅行是那种没有任何预先计划的自发旅行。