I was merrily spelunking through my imagination last night. It was a good trip because I came away with a lot of good ideas and plans for the next novel. If you've tried the game called Minecraft--my next comparison is inspired by this iconic game (if not, Minecraft is a computer game where you mine in search of rare minerals).
Thinking about the story and the world it is set in is like mining. Sometimes you just find ore, but sometimes you strike gold (or even obsidian). I never know what a "mining expedition" is going to produce. I rarely come away without some new idea or insight, but sometimes I hit a vein and am awash in precious ideas. This is where the mine cart comes in: I have to write these ideas down as quickly as possible, because my memory isn't always the greatest.
It's such a pleasure to explore a story and an imaginary world like this. I picture myself moving through these imaginary locales like a director on a movie set. I watch the scenes play out in my mind. At their best, the stories that result are like field journals. I hope that the joy that I feel when I create these adventures shows through in the final product.