Shooting time-lapse video is simply finding a way to speed up time. One of the simplest ways is to get a remote switch for a standard still camera. By taking these shots at a particular interval and then piecing them together into a movie, you can give the allusion of time speeding up.
Clouds are one of the easiest things to do time-lapse video with. They're around us every day and the move quickly! You can set a camera up and within 15 minutes you have a nice timelapse. Here is a nice selection of clouds I found.
Flowers a more difficult than most things because not only do you have to figure out the logistics of the camera, you have to make sure the flower has the right conditions to grow! Here is one of my favorite compositions web.
Maybe I should call this section "decomposition" because the organisms are already dead. Watch these.
For more about timelapses we recomend this article: Time Lapse Video Secrets: HD and Beyond with your Digital Camera