Here's my favorite design for a jack-o-lantern; it's a (tiny) mini-pumpkin carved with the circuit diagram symbol for a light-emitting diode. Naturally, it's lit by an LED.
This LED circuit inside this one is a TireFly, which is a type of blinky light designed to mount on the tires of a bicycle. You can get these things at regular hardware stores or sporting goods stores-- I got mine at Target-- and so this is one of the easiest and quickest ways to get an LED in your pumpkin.
Read more about this pumpkin and its circuit here.