THINGS YOU CAN DO WITH ABSOLUTELY NOTHING

Blink wildly and then close your eyes really tight for an interesting light show
(Amusement Potential: 1-5 minutes)
See a variety of blobs, stars and flashes. Try to make out shapes and see if your subconscious is trying to send you a message (perhaps that funny shape is saying, 'send all your money to urban75.com'?)

See how long you can hold a note
(Amusement Potential: 4-20 minutes)
Not that much fun, but it sure passes the time. Play with a friend, or try to beat your own personal best. Inhale deeply and then try and make a noise for as long as you can. Earn extra points for making your partner laugh or ending on an amusing note.

Try to not think about penguins
(Amusement Potential: 1-5 minutes)
This is especially hard, because by trying too much, you remember what you were trying to avoid thinking of. If you try too little, you end up thinking about penguins anyway.

Use your secret mind power
(Amusement Potential: 5-10 minutes)
Pick a passing by and try to use your mind power to command them do something, like drop their bag or knock into someone. The law of averages dictates that sooner or later one of your mind commands will come true, so you can convince yourself that you really have super human powers and waste even more time trying them out.

Pretend you're a robot
(Amusement Potential: 1-3 minutes)
Walk down the street with mechanical movements, adding 'zzzzzt' sounds with each motion. Pretending to have a motor broken in, say, your left hand can add at least 30 seconds more entertainment.

Scratch yourself
(Amusement Potential: 1-3 minutes)
Go ahead, scratch yourself now. Even if nothing itches, go ahead. Doesn't that feel pretty good?

Rate passers by
(Amusement Potential: 10-15 minutes)
Secretly award passers by marks out of ten as you go along, offering (unsaid) expert criticism over their clothing, hairstyle and footwear choices.

Repeat the same word over and over until it loses its meaning
(Amusement Potential: 1-3 minutes)
Pick a random word out of a magazine and say it aloud to yourself until it becomes a meaningless set of noises.

Pinch yourself
(Amusement Potential: 1-3 minutes)
What is pain? Why is it unpleasant? There's nothing physical about it - it's all in your mind. Plus, after pinching yourself for awhile, boredom will seem nice next to being in pain.

Try to swallow your tongue
(Amusement Potential: 1-2 minutes)
There's not much to say about this one. It is possible, but really stupid.

Pretend to be a car
(Amusement Potential: 5-10 minutes)
Make appropriate revving noises in your head as you walk along and add a racing commentary as you pass strangers in the street. Use blinking eyes as indicators for extra authenticity.

Make Star Trek door noises
(Amusement Potential: 1-2 minutes)
Stand by an electric door to a bank or something and make that silly "Scccccccchwop" sound heard whenever people popped on to the bridge to hang with Captain Kirk.

Look at something for awhile, shut eyes, study after image
(Amusement Potential: 2-5 minutes)
Another great time waster. It takes about 30 seconds of staring to create an after image, and the image is then viewable for about the same length of time. Fun to combine this one with pushing on your eyes.

Get yourself as nauseated as possible
(Amusement Potential: 5-10 minutes)
Best achieved by looking straight up and spinning around. Try to be so dizzy you can't even stand up. This is also entertaining due to the "makes boredom seem a lot better" effect (see "Hurt Yourself").

Invent a weird twitch
(Amusement Potential: 5-10 minutes)
Adopt a bizarre twitch (e.g. flicking your head irregularly, twitching with eye or busting out sporadic cough noises) and try it out when you go shopping.

Make a low buzzing noise
(Amusement Potential: 15-30 minutes)
Hours of fun in libraries! Keeping a totally straight face and looking nonchalant, make a low pitch humming/buzzing noise and see who reacts.