I have a few to add on...
Every time you see someone, simply poke them in the side while making a buzzing noise.
Walk up to everyone around you and put a magnet up to their arm, saying that you were checking to see if they were magnetic or not.
While at someones house, continously cause their computer to lock up and shout their name and make them fix it.
While eating dinner with someone, bite your fork / spoon firmly so that when you pull it out of your mouth it makes a metal-on-metal sound.
Go to your local McDonalds and take all the sugar packets that they have in the little station with the catsup, pepper, salt, etc.
Make a clicking noise while someone is trying to talk to you.
If you are in a large bulding with lots of people, randomly start shaking and shout "They are coming! Everyone hide!"
Talk to everyone with an extremely loud voice.