This kit can be quite an attention getter.  Although it keeps time just like any other clock, the outputs are all coded into binary and displayed as 4-bit codes.  Only those who know binary can read the time.

Components:  40 resistors, 6 capacitors, 10 diodes, 24 dual color LEDs, 8 ICs, plug-in transformer, 4 push switches

Principles taught:  power supplies, binary numbers, large scale digital logic