They stayed there only one night and set ____ the next morning in pursuit of excitement and adventure. (up or off)

根据句子结构和语境，这里需要在“set”后填入一个介词，与“up”或“off”搭配。结合上下文，“set off”表示“出发”，更符合“第二天一早出发去追求刺激和冒险”的语境。而“set up”意为“建立、搭建”，与句意不符。因此，正确答案是“off”。

完整句子为：
They stayed there only one night and set off the next morning in pursuit of excitement and adventure.

答案：off