I can only get really vivid daydreams when I'm just about to fall asleep. I can remember being on the toilet once and I was so close to falling asleep every so often I would start to fall off the toilet but then I'd realize it and correct myself. While this was happening I think I was having Hypnogogic imagery, but I was able to control it somewhat. I remember one image was this guy who would bring out tools from behind his back, and I was able to start changing what he would bring out to different objects.
I had to be really tired for this to work and be as vivid as it was though...