A couple of months ago I came across Greyscale Gorilla - a blog dedicated to all manner of motion graphics and 3D design.

Whilst traversing the wealth of posts over there I found out about the "5 second projects" - basically they provide you with a theme and all you have to do is create an animation based on it. The animation can literally be anything you want it to be as long as it has something to do with the theme they set.