Evil mad scientist has  a great project for the electronic do it yourselfer.  The concept is that a pair of yellow LEDs are placed in an otherwise inconspicuous pumpkin, and a photo sensor allows the circuit to turn on the lights at night. 

The circuit itself is very straightforward, incorporating minimal components.  There are two schematics listed on the project site, but we suggest using the one above with an adjustment pot so that you can adjust the lighting for your given conditions.

In the project, which definitely looks to be fun, they drill holes for the LEDs.  I would suggest trying to drill a whole partially through the pumpkin, leaving a thin layer on the outer skin so that in the daytime there is no hint of the creepy effects that come out at night.  Plus it is a safe effect to have on, no power cords or candles required.

The Evil Mad Scientist project website includes step by step details, so we suggest going to have a look see if you are interested in creating such a project.