Here are a couple of projects for a mechanically scanned clocks. The idea is that you spin some LED's and by flashing them at the right instant you can make letters or numbers. Using the fact that we have persistence of vision, meaning that we see images in our mind for a fraction of a second after the object isn't there. These kinds of projects have made their way into some toys and gadgets now too.
Bob Blick's Mechanically Scanned Clock
Ken's Propeller Clock
I've often wanted to build one of these. My idea is similar to the second one except that I don't want to write the letters, I just want to show three hands and twelve little dots. In other words I want to make it an analog clock. Maybe someday.
I couldn't find the details of this one but it is seriously cool...
