Continue to complete your own version of your story about the underground world as you progress through the passages, adding various moments where you can collect and drop inventory items. You can even add dead ends where a dropped item causes some problem, and the passage has to be restarted. Be as creative as you like, remembering that we will be adding a way for the collection of some keys to be contingent on the collection of certain items in the next chapter, so plan your story accordingly.