One of the things I am doing for Halloween involves a pumpkin, a proximity sensor, and a music shield. Unsuspecting suspect comes near pumpkin, and the pumpkin talks to them. This is a very simple item, but one that can be pretty fun and very useful.
The Arduino is below. This is based on an Uno and uses a music shield and a proximity sensor. The music shield is from Seeed Studio and is the most expensive part (save the Arduino itself). The proximity sensor is a Sharp sensor, which can be bought from several sources. One potential addition may be a small op amp as an audio amplifier.
The code is on Github.