Interactive Halloween props are always fun and they are surprisingly easy to make. This year I designed a simple system that rotates a skull to face you and follows you movements as you walk by.
To accomplish this I used light sensors to detect a person's shadow. An Arduino microcontroller then calculates where they are standing and activates a servo that turns the skull to face them. When they move, the skull turns to follows them.
The first thing that you need to do is select a Halloween prop that will turn and follow people walking by. Look for props that are both creepy and light weight. I chose to use a foam skull but there are a lot of other props that can also work. Use your imagination.