让我们去散步吧   此句没有主语 This sentence doesn't have a subject.

男：让我们去散步吧！  the same as above
女：我们去哪里？
男：现在很早，所以我觉得下去外滩   :下海滩去。下...去 is a fixed collocation.
女：好
男：你吃早饭了吗？
女：对，我已经吃了蛋饼
男：好，我还要吃了，所以我吃了，然后我们走吧   "了" indicates that the action has been done, "还要" shows that eating is something you want to do next which means you haven't eaten. It is contradicton.
女：太好了！