If you have a few mousetraps laying around, you can use them to create electronic pressure sensors that can set off a wide array of electronics. The spring mechanism is already in the mousetrap, so all you need to do is create terminals where a circuit can be completed. This is the perfect combination of engineering and pranking, all in a fairly quick and easy project. Now, you can have a car horn be triggered when your friend comes to pick up his backpack. Muahaha.