Blinking Christmas tree for Arduino Uno (kit of parts incl. power supply + Uno R3 board)
This highly seasonal kit is easy to assemble. All required parts are supplied, including a power unit and a Uno R3 board.
The construction is fun, it has a little bit of mechanics (no tools required) apart from the inevitable soldering iron and cutting pliers. The steps of this realization are adequately described in a richly illustrated assembly manual.
All that remains is to program the flashing of the 40 LEDs with the supplied Arduino sketch. If you wish, you can modify the program to customize the blinking sequence.