I'm (re)drafting the beginning of a novel, and I keep coming across completely different stories I could be writing with the given setup. Totally different themes, outcomes, character arcs, all from this point. All I'd have to do, is make a different choice here, or here or here.

This works best with a largish writing group - at least eight people. You can do it by yourself, but it's a lot of work, and probably not as much fun.

First, come up with your starting situation. A character with some quirks, dreams and flaws. A situation he or she is in. A conflict.

Now, come up with a decision your character will make. A major decision or action - not icecream flavour, unless the other flavour is about to give her food poisoning. A plot point, basically - something that will change the course of the story, and even the emphasis - one way, it's a coming-of-age story about a girl with beans, the other it's a crime-solving mystery. Split your group - one half of the group takes one path, the other takes, well, the other.

Now, separately, decide where this choice has lead your hero/ine. What's the conflict now? You might have to plot a little along the way; it's doubtful that major life changing events crop up every five minutes. Again, a change that will alter the story (though it'll do so less significantly than the first one - still try for a major difference). Again, split your group, half each way. As both of the original groups have done this, you now have four story paths.

Continue making decisions and splitting until each 'group' consists of one person. That means each person has a collection of plot points and decisions to write a story about the same character from the same starting situation. 

Now go write. Everyone's working from the same character and the same starting point. But different plot decisions make the character journey totally different. The comparison by the end can be startling.