What every Christmas tree needs are LIGHTS! Inspired by pauloricca's LED Christmas tree from back in 2009, (thing 1406), it was hollowed out, a mounting hole added and a base designed to enable me to string a number of these together after fitting a colour changing LED to each one.
The LEDs are widely available - and if four of them are wired in series, the set can be powered by a standard 12V DC plug-in mains adapter. As a set of 4 LEDs will only take around a total of 120mA, a mains adapter rated at 0.5A or more will easily cope with having four sets of them in parallel which is what I have done. (Photos show just three sets in parallel).
Each set of four lights requires four hollow Christmas trees, four stands and four LEDs. You can use whatever flavour of LED you like but my choice were the slow flashing rainbow type.
You will also need connecting wire, heat shrink tubing to cover the soldered joints and connector pieces to join one string of LEDs to the next set. (As well as to the 12V DC power source).
Wire the LEDs in series, bending the LED leads and pushing the wires into the groove underneath the stand. My wires were roughly 100mm long. Each set will also need two more wires through the grooves - one for the negative supply and a second for the positive. If you use wire colours other than the standard black and red, make sure that you note down which is which as you will destroy the LEDs if you wire them the wrong way round. (I took the opportunity to use wire out of my scrap box).
The first set of lights needs to connect to your power supply - use a DC plug which matches the DC socket on the mains adapter. The end of this first string needs a socket so that it can supply power to the next set. I used standard 0.1" crimp terminals. From then on, apart from the last string, you need power in and power out connectors. The final string only needs power in.
Check all your wiring before connecting any power and once you are happy, I suggest you test them one at a time.
Thanks to NumberSix who provided the idea and then helped me to learn some of the skills needed to produce this thing in Sketchup. He also suggested that I print the Christmas trees using the spiralise feature in my slicing program. This feature prints a single shell by continuous movement of the Z axis which is ideal for hollow objects.
The Christmas trees have a "normal', (not spiral), 0.5mm base and once that has printed, the spiral begins. If your slicer does not have such a feature just go for a single shell. The trees push fit onto the bases. My bases were printed in different colours - red, green, gold and black.
I have now included an alternative STL for the hollow Christmas tree with a 10mm hole in its base should you have problems slicing the first version which I uploaded .