Laser pointers (and low-power laser diodes) are cheap these days. It'd be fun to build a clock that used lasers mounted on rotating drums for the hands: a red laser for hours and a green laser for minutes, just projecting dots onto a wall. Given multiple lasers or some careful use of beam splitters, you could create a circle of green dots with one red dot to act as the hand, or similar. (Obviously you could project anything you like given controllable mirrors, but I'm after something with less mechanical complexity.)