Something like a matrix orbital would be a cool option. Then you could have a bunch of stuff plus the clock. I want to build a nixie tube clock to fit in a couple bays, but they heat will probably be equal to another cpu or more.
You could always just buy a 5.25" bay LCD display and program it to only show the clock... the cool thing about this, is that it could be in whatever size and font you'd like. Actually you could put just about anything you wanted on the screen.