This project was a bit of a rush job in order to get it finished before a Halloween but it turned out great and was allot of fun. The jack o' lantern was made of paper mache and a few other bits and pieces, painted with acrylic paint and had 15 red and yellow 5mm LEDs inside that were switched on and off randomly with a microchip. I'll explain briefly how I did it all below. Click the thumbnails to see a larger image.
I used a balloon to create a hollow form. Strips of newspaper were applied over the balloon using a paste made from flour and water. The mache balloon was placed on a plastic sheet so it wouldn't stick to the work surface as it dried. I tied strings tightly around the balloon to create segmentation lines, applied more paper over them and then I used an old broom stick wedged against the ceiling in my shop to create an indentation on the top.
The mache pumpkin took several days to dry completely. Once dry, I glued a disk of thin plywood with a wire for hanging the pumpkin into the indentation on top of the pumpkin. I sandwiched the wire between two pieces of foam shaped like a stem and then applied more newspaper over the foam stem lapping it onto the pumpkin to secure everything in place.
I used a PIC12F675 to switch 5 LEDs randomly to simulate a candle flame. This did not produce enough light for what I wanted to do so I set the circuit up to switch 15 LEDs. I soldered together 5 sets of three 5mm LEDs each in parallel and used a 7.5VDC wall wart from the junk box to power the circuit. Maximum voltage for the microcontroller is 5V so I dropped it to ~4.5V with an LM317 I had lying about. The fourth image below provides a 10 second video of the the circuit operating in the dark.