Make a loud noise of hitting the same key over and over again until you see that your neighbor is noticing. Then look at your neighbor's keyboard. Hit his/her delete key several times, erasing an entire word. While you do this, ask: "Does your delete key work?" Shake your head, and resume hitting the space bar on your keyboard. Keep doing this until you've deleted about a page of your neighbor's document. Then, suddenly exclaim: "Well, whaddya know? I've been hitting the space bar this whole time. No wonder it wasn't deleting! Ha!" Print out your document and leave.
Remove your disk from the drive and hide it. Go to the lab monitor and complain that your computer ate your disk. (For special effects, put some glue on or around the disk drive. Claim that the computer is drooling.)
Stare at your neighbor's screen, look really puzzled, burst out laughing and say "You did that?" loudly. Keep laughing, grab your stuff and leave, howling as you go.
Point at the screen. Chant in a made-up language while making elaborate hand gestures for a minute or two. Press return or the mouse, then leap back and yell, "COVEEEEERRRRR!" Peek up from under the table, walk back to the computer and say "Oh, good. It worked this time," and calmly start to type again.
Keep looking at invisible bugs and trying to swat them.
See who's online. Send a total stranger a talk request. Talk to them like you've known them all your lives. Hang up before they get a chance to figure out you're a total stranger.
Bring a small tape player with a tape of really absurd sound effects. Pretend it's the computer and look really lost.
Pull out a pencil. Start writing on the screen. Complain that the lead doesn't work.
Come into the computer lab wearing several extra-stinky species of flowers in your hair. Smile incessantly. Type a sentence, then laugh happily exclaim, "You're such a marvel!" and kiss the screen. Repeat this after every sentence. As your ecstasy mounts, also hug the keyboard. Finally, hug your neighbor, the computer assistant, and then walk out.
Run into the computer lab, shout, "The Apocalypse is here!" then calmly sit down and begin to type.
Quietly walk into the computer lab with a Black and Decker chainsaw, rev 'er up, and then walk up to the nearest person and say, "Give me that computer or you'll be feeding my pet crocodile for the next week."
Two words: Tesla Coil.