I'm gonna show you a very easy to make decorative object which uses one single RGB color-changing LED.
Step 1: Materials
-A cheap crystal ball , the ones with a flat side which is used to stand still , in our case it's a good space to place the LED in. I got my crystal ball from Ikea .. (Tip: use a ball with bubbles in it , like this one http://farm3.static.flickr.com/2188/2238715683_ff8a6a16f6.jpg , so the LED light can shine in the bubbles)
-An automatically color changing RGB LED (The ones with only two pins , which change their color automatically)
-A Jack male cable from some old headphones
-A Jack female connector
-A battery holder which holds as many batteries as your LED needs (in my case 6V so i used a 4AA battery holder)
-A 5 Liter bottle cap
Hot glue gun
Step 2: Step 1: Put the Led in Place
You will see that a headphone jack cable has actually 2 cables glued together , you may want to cut one of the cables as you won't need it.
Remove the protective plastic from one of the two cables and then get a ligher and burn the protective layer from the orange cable , so you will be able to solder on it.
After preparing the cable , solder the two wires to the LED pins , and then use hot glue to put the LED in the center of the cap.
Step 3: Step 2:Glue the Cap to the Crystal Ball
Find the flat side of the ball and place it on the cap , and then use hot glue to keep it in place.
Hot glue is very good for this because if you want the ball back as it was before , just pull the cap very hard and it will come out,leaving no glue marks . Additionally , clean the remaining glue from the ball.
Step 4: Step 3: Polarity Check
Now use your power supply's wires to check all the 3 connections on the female jack and find the correct ones which will light up the LED . Solder the wires in the correct places on the connector and then use the plastic part to twist it around the connector , so now it's isolated(if you solder without the wires coming out of the plastic isolation , you will have to cut it to put it back).
Now put batteries in your battery holder and it's all done ! The next time you want it to light up just connect the two jacks together.
Now you may notice that i used a 9V battery clip from an old AC/DC adapter for my supply , because my battery holder was using a 9V clip .