Actually, this isn't a very good idea. For those of us in the northern hemisphere, Christmas comes in winter, which means short days and less chance of getting a full charge on the batteries. Also, the solar cells can be covered with snow and not generate any charge. Finally, if an ornament is hanging on a tree, it will be in the shade most of the time anyway from the body of the tree.
However, just because it's not a good idea doesn't mean it shouldn't be done. Imagine what our world would be like today if people like Doctor Frankenstein said to themselves, "maybe this isn't a good idea."
In order to get this to work, you would need to locate the solar panels on a board or something away from the actual Christmas tree and connect it to the ornament via some thin wire, so that the shade won't be a factor. Also, since I live around 42 degres lattitude, I should incline the board about 48 degrees from vertical to face the solar panels toward the sun (actually maybe 55 degrees or so,since the earth's angle has tilted even further away from the sun in winter - isn't it about 9 degrees precession or so?) Anyways, the snow should slide right off in that case. Finally, I don't need a full charge really - just enough to light up about 4 or 5 hours should be enough to be festive while everyone is awake, no one will miss the Christmas lights at 2 AM.
Fortunately, there are such rechargeable solar lights readily available, for use as walkway lights around your home. There are many Intructables which show you how to re-purpose these devices for various things, and they inspired me to use walkway lights for ornaments.
As always for an instructable like this, you will be using tools that could burn you or cut you, so be cautious if you are unfamiliar with them. This will void the warranty on the lights you buy, so if the light doesn't work afterward for some reason, you can't get your money back, and I can't be responsible for damages or injury you may get. Just remember - pain is fleeting, awesome is forever.
Step 1: What you will need
The ornaments I will make keep the solar panel, battery, and LED circuitry module in one piece, and add a long "extension cord" to the LED itself, so the LED can illuminate an ornament of some kind and still be far away from the solar panel. With this model of walkway light, it's a bonus that the little clear shell easily detaches and makes its own ornament, so you can add a long wire to the LED and re-assemble the original clear shell around the LED, so that the ornament practically makes itself. If you use a different model walkway light, you can still move the LED away from the solar panel, but you may need to get (or make) a transparent ornament for the LED to illuminate.
The LED is connected to the circuit by its own leads, which are about 2cm long. That means that you don't need to desolder anything to get this to work - just clip the wires to the LED and splice in 3 or 4 meters of wire , so the LED is far away from the solar panel.
So, in addition to the walkway light(s) you will need some thin comm wire, like CAT-5 or something. I found a 100-foot modular telephone cord for $1, so that's what I used in this case. The phone cord has 4 conductors, so I could use one length of wire for two walkway lights. The stuff was cheap, but it's hard to strip and not easy to solder, but it did the trick.
You will also need some stiffer wire, like bell wire or steel picture-hanging wire to hang the final ornament from the tree.
Because this will be outdoors, you will need silicone caulk to weatherproof it. I also decided to try some Liquid Electrical Tape this time instead of my usual black electrical tape, but quite frankly I would not recommend it - just go with electrical tape.
As far as tools go, you will need a soldering iron and solder. If you are not familiar with soldering, there are several Instructables posted about soldering techniques. You will need some pliers and a wire cutter/stripper. You will need a drill and screwdrivers.
Step 2: Disassemble the walkway light
Pull the stalk from the clear shell. The stalk will not be used for this project, and can be discarded or used in another project.
Open up the top cap, exposing the circuit and battery. Note the little plastic strip in the battery holder to prevent the battery from discharging in the store. Don't forget to remove this later!
Cut or melt the plastic tabs that hold the circuit board to the plastic case, and pull the circuit board away from the case (don't break the wires).
Pull the LED out of the plasic case so that the two halves of the plastic case can now be separated. The LED is held in place by a blob of hot glue, and should just pop right out.
When you are done, you will be left with the clear shell, the top of the top cap with the battery inside it and the circuit board and LED dangling from it, and the bottom of the top cap which has a hole where the LED was.
Incidentally, even though each walkway light I had was similar on the outside, nearly every one was different on the inside - different models of LEDs, different styles of circuit boards, different internal layout. The only real similarity inside was the battery and the plastic tab to protect it.
Step 3: Splice extension wire for the LED
Drill a small hole at the 6 o'clock position.
Feed the LED extension wire through the hole at the 4 o'clock position, so that there's about 3cm of wire sticking through the other side, and tie a big knot in the wire so that it can't pull back through the hole.
Clip the leads to the LED, separating it from the circuit board. Here's a tip - whenever I cut leads like this, I always cut on one lead close to the circuit board, and the other close to the LED. This way, the LED has one long and one short lead, and the circuit board has one long and one short lead. When you splice the new length of wire between the LED and the circuit board, the wire from the LED's long lead goes to the circuit-board's short lead, and the LED's short lead goes to the circuit-board's long lead. This way, it's nearly impossible to get the polarity wrong on either end of the splice, since the length of the leads will tell you which wire goes where. Also, when you tape up the splice with electrical tape, the wire's cuts are separated and won't short out - the insulation from one wire is always next to the bare splice of the other.
Splice in your 3-4m extension wires and solder them.
Step 4: Mount the solar panels and batteries
REMOVE THAT LITTLE PLASTIC TAB FROM THE BATTERY HOLDER!
Use silicone caulk to seal around the edges of the solar cell cases, especially where the wire leading to the LED sticks out.
Set the board of solar panels aside to dry (it needs a whole day to set up and cure).
Step 5: Mount the LED in the case
Insert the LED through the hole, so that it is positioned exactly where it was when you bought the walkway light.
Shoot in a blob of silicone caulk behind the LED for weatherproofing.
Set this assembly aside to dry and cure for the rest of the day.
Step 6: Final assembly
Secure the clear plastic shell so that it again covers the LED.
The entire system is now complete and ready for hanging! If you do not like the way that the bare wires are sticking out of the top, you can add your own ornamental decorations to it, or dangle a ribbon or bow from the bottom or something. As it is now, the ornament will glow white. It's possible to use a different colored LED in this circuit, but a different LED may use more or less power than the original, so there's a chance it will discharge more quickly (especially if you are using a super-bright LED, or an LED that changes colors by itself). You could also try wrapping the ornament in different colors of cellophane to get different color effects.
For now, for the sake of simplicity and cost-savings, I will use the same LEDs that came with the system and just enjoy white-light Christmas ornaments.
If you can, use wires that have dark-colored insulation (get black telephone wire). That beige wire I used is very difficult to hide in the tree during the daytime - that's the reason that Christmas tree lighs have dark-green wire connecting them up.
Merry Exabopper Xmas!