impromptu

他即兴弹奏了一首钢琴曲，让观众惊喜不已。

He played an ___impromptu piano piece that amazed the audience.

新郎的弟弟在婚宴上站起来即兴发表了一段祝酒词。

The groom's brother stood up and gave an ___impromptu toast at the wedding reception.

邻居们闻到烧烤的烟味后纷纷自带食物过来，一场即兴烧烤就这样开始了。

An ___impromptu barbecue broke out when neighbors smelled the grill smoke and brought their own food.