If you want to scare some kids as they approach your house to get some candy on Halloween have a look at this project by GreatScott. He uses an Arduino which has an ultrasonic sensor connected to it. The sensor detects when someone gets into the scare zone. To scare the curious kid some LEDs light up the pumpkin and a buzzer sound. This idea could be adapted many ways, I could imagine having a hanging skeleton in the corner of the porch and a large vibrating motor behind it which comes to life when you approach. If you are interested in building this you can find the file here.