I took my cue in a moment, and did the only rational thing there was to do. 

I reached the note towards him, and said, carelessly: "Give me the change, please."

这里的and did the only rational thing there was to do. 中：there was to do 不明白。

如果是there was， 那是定语从句。

there was to do 是否也是定语从句？如果是，怎么理解呢？do代表了什么呢？

there (was to) do 麻烦老师分析一下，谢谢！