Clocks use a variety of circuit blocks - you need something to keep time; you need something to display that time; you can add other features.

With just "keep time" and "display time" you still have a variety of different methods. You can count mains frequency; you can use crystals and dividers. Displays can be 7 segment LEDs or LCDs or binary or analogue motors. Each require different approaches.

Even if it's just a shop-bought off the shelf kit the student learns a bit of component identification and handling and some soldering skills.