Always carry a camera flash unit in the car. Drive 10 K's below the speed limit until someone comes up behind you, wait for them indicate and then speed up to exactly the limit. They'll feel inadequate and stupid if they don't overtake you from this point, since you're obviously an irritating Sunday driver, so they'll have to speed to get past, at which point you stick the flash out and flash them. If it all goes to plan, they might even panic and go back to see if anything was there. Alternatively, try to master the art of changing lanes without running over any reflector lights.
There are always lots of buses and trains around- try catching one at random and finding out where it goes. Get off somewhere and go for a long walk in an unknown suburb. Go to corner delis and buy aspartame (!NutraSweet et al) sweetened beverages that are right at the back of the fridge and marvel at how bad they taste after they've been sitting in the same spot for years and years (true story- I went to a shop in an unknown suburb after catching a random train and found a Pepsi Max can with a three year old competition on it) Consider legal action, as drinking formaldehyde is never that pleasant. Select a random product and go to all the shops that sell them and gather information. Challenge the shopkeepers on their prices, demonstrate above-average levels of knowledge regarding it, ask why you should buy from them and not someone else, inquire about bulk discounts (especially for things that you'd rarely need more than one of, like cars or boxes of tissues) Ascend the tallest buildings available and look out of one of the windows at the top. Challenge your friends to see who can stay in the longest before an irate business person becomes suspicious and kicks you out. Go to news agencies and bookshops and indulge in some free reading, but walk around the shop so that it looks like you're not just reading the same thing. The ability to speed-read is handy, or even better, take pictures of interesting articles with a camera.
If your school has a computer room, cause mayhem by changing default Windows programs around with a resource editor- change standard menu items and login screens to contain amusing messages. If there are intercom phones in your classrooms, rewire them so that you can cause them to ring by remote control (as done by Nicholas! :) If you are in a boring class and have an inattentive teacher and are sitting near the back of the room, jump through a window and get a friend to pass your things out. If it's a large classroom with a significant empty space at the back, sit in the same desk and gradually move it back each lesson. When you're on the wall, try rotating it a bit more each lesson. See if you can manoeuvre it to be facing the opposite way to everyone else without getting caught. If you sit next to a window and have friends with free lessons at the same time and are situated near a supermarket, arrange a smuggling racket. Go around to everyone and get orders. Inflict a service charge. If you have unsupervised free lessons, while your friends leave temporarily, block any entrances with large stacks of furniture. Write your own name on desks and blackboards- if you get in trouble, tell them that you wouldn't be stupid enough to write your own name (unless you obviously ARE) Don't phone in bomb threats. That's lame.
Call the police and tell them that you suspect they're a dealer. Get various people to call them up and leave messages on their answering machine that go something like "I need five t-shirts. Same time and place as last time?" so that the police phone taps would get them. Ring them up and work and ask if they can come over to pick up some documents that need to be typed up, but when they come over apologise that you forgot to bring them, but give them $20 for their trouble anyway. Hand it over sneakily so that the police lurking in the bushes with video cameras will think that a transaction has been made. Put an illicit substance in their coffee, and attach a package of some more to the underside of their car with in one of those hollow magnets designed for keeping spare keys. Eventually they'll get busted by the police, but traces will be detected in their blood. (This elaborate scheme was concocted by Imlay- I hope he didn't ever actually do it :)
While on a tour of the Flinders University Faculty of Science and Engineering during the course selection in year 12, a lecturer asked our group if anyone knew what a particular piece of apparatus was. I correctly identified it, from two metres, as an evaluation board for the Motorola MC68HC11E9 microcontroller.
The second most impressive thing I've ever done was when I was tying my shoelace up, and was distraught to find that it broke in my hands. I fixed it with the brand new spare shoelace that I happened to have in my pocket.
An unbreakable toy is useful for breaking other toys with.