He showed me around the city, which I thought was a generous gesture.

which 在这里是关系代词，引导非限制性定语从句，指代前面整个主句的内容（即"他带我参观了这座城市"这件事）。这种用法中which不指代某个特定名词，而是指代前面整个句子或部分内容。