I made some Grumpy Pumpkins (several versions on Thingiverse) out of natural and orange PLA, then taped them on top of a strip of addressable RGB LEDs controlled by an Arduino.
Cheap and fun. I’ll probably give the pumpkins away to kids with really good costumes. The LED strip comes right out.
http://www.youtube.com/watch?v=Cc4oJ-pwhOc
That is freaking awesome