We created a hollow version of the Sydney Opera House and installed Adafruit NeoPixels underneath. Printing the opera house with a special light diffusion PLA filament makes the individual LEDs disappear to create an amazingly smooth illumination. We've tried similar things with transparent and natural PLA and it doesn't work nearly as well.
Read the blog post about this project
https://www.youtube.com/watch?v=1BmSs7Mlp7w