Check out Daniel Andrade’s binary LED clock built using the Arduino. It’s well thought out, including controls for setting the time and hibernating with the LEDs off. Each hour and minute digit is represented in binary form, so it’s actually fairly easy to read once you get accustomed to it.
The circuit and source are available from Daniel’s site. If you’re ahead of the game and already thinking about what to do this Saturday afternoon, this might be a fun option to add to the list.