Sooner or later most enthusiasts will experiment with RGB LEDs for a while then move onto something else. However if you can rustle up a box of plastic spoons and an empty water bottle - why not make something useful and more permanent instead? A perfect example of this is by Instructables user "LostRite" who shows how to make a decorative lamp enclosure that can be illuminated via an RGB LED controlled by an Arduino. It's a simple yet visually-appealing device, for example:
If you're looking to make more creative designs with LEDs, consider the Freetronics RGBLED: full colour module. It includes a bright RGB LED on the top of the board and a WS2801 constant-current, addressable, multi-channel LED driver on the back. This smart module can be daisy-chained, so you can connect a number of these together in a string and drive each of the module colours individually from your microcontroller. For more information and to order, visit the product page.