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. Best to do this in the privacy of your own company or people will think you're weird, which you definitely are.
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 mate, 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 rabbits (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 rabbits anyway.
Use your secret mind power (Amusement Potential: 5-10 minutes) Pick a passer 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.
Scratch yourself (Amusement Potential: 1-3 minutes) Go ahead, scratch yourself now. Even if nothing itches, go ahead. Doesn't that feel pretty good? Err ok moving on...
Repeat the same word over and over until it loses its meaning (Amusement Potential: 3-5 minutes) Pick a random word out of a magazine and say it aloud to yourself until it becomes a meaningless set of noises. Then try and hold a conversation using that word as much as possible. Brilliant!