Adjust the tint on your TV so that all the people are green, and insist to others that you "like it that way".

Staple papers in the middle of the page.

Change channels five minutes before the end of every show.

Produce a rental video consisting entirely of dire FBI copy warnings.

Write the surprise ending to a novel on its first page.

ONLY TYPE IN UPPERCASE.

Sit in your front yard pointing a hair dryer at passing cars to see if they slow down.

Write "X - BURIED TREASURE" in random spots on all of someone's roadmaps.

Sing along at the opera.

When you're on an elevator, and someone else gets on, turn around and stare at them instead of the door  (Provided by The Clueless Gamer)

When getting on a bus and only 1 other person is on it, go sit in the seat right next to them
(Provided by The Clueless Gamer)

Inform everyone you meet of your personal Kennedy assasination/UFO/OJ Simpson conspiracy theories.

Drive half a block.

Buy a large quantity of orange traffic cones and reroute whole streets.

Change your name to John Aaaaasmith for the great glory of being first in the phone book. Claim it's a Hawaiian name,  and demand that people pronounce each A.

Construct elaborate "crop circles" in your front lawn.

Give a play-by-play account of a person's every action as they are doing it.

Honk and wave to strangers.

Holler random numbers while someone is counting.

Decline to be seated at a restaurant, and simply eat their complementary mints by the cash register.

At a golf tournament, chant "swing-batatatatatata-suhWING-batter!"

Stand over someone's shoulder, mumbling, as they read.

Ask the waitress for an extra seat for your "imaginary friend."

Ask 1-800 operators for dates.

Repeat everything someone says, as a question.

Signal that a conversation is over by clamping your hands over your ears.

Set alarms for random times.

Tie jingle bells to all your clothes.

Finish the 99 bottles of beer song.

Leave your turn signal on for fifty miles.

Name your dog "Dog".

Repeat the following conversation a dozen times:
      "Do you hear that?"
      "What?"
      "Never mind, it's gone now."

Ask people what gender they are.

Follow a few paces behind someone, spraying everything they touch with a can of Lysol.

Make beeping noises when a large person backs up.

Chew on pens and pencils that you've borrowed.

Mow your lawn with scissors.

Invite lots of people to other people's parties.

Stare at static on the tv and claim you can see a "magic picture."

Learn Morse code, and have conversations with friends in public consisting entirely of "Beeeep Bip Bip Beeeep Bip..."

Inform others that they exist only in your imagination.

Reply to everything someone says with "that's what YOU think."