Wow, Celtic Memories, sorry but your idea is not unique. Ever since I was seven, I've been doing just what you talk about, and now I turn those cute little fantasies into (I find) wonderfully entertaining stories. Another method I use is ffing, or rather FanFicing. I never do normal fanfics, so I tend to invent new stuff, which I later find can be part of a stand-alone series. I never write oneshots... oops, there's my FF background talking. I mean I rarely write singel stand-alones for fantasy/sci-fi stories.
Anyway, letting a scene run rampant in your head is one of the best ways to get in the zone(for me), even if you use very little of the original scene.
I guess this is sort of late, but I never seem to find forums that are as active as my first one, so most of the topics are a few day old. Oh well.