DO NOT do this around where you keep your laptop, monitor, disks and such. the magnet is strong enough to cause potential problems. CRT monitors are particularly prone to long term damage from magnets.
We have all seen the magnet toys where you have a base, and then a bunch of nuts or metal pieces you can sculpt into interesting shapes. This Idea-able is an easy way to replicate that science experiment in your cube, and keep you neighbors wondering. (In the case of my neighbors, it's more a matter of an eye-roll and a silent thank you prayer that I am not blowing something up or starting a fire *again*)
1 dead hard drive
various tools to crack that baby open
hand full of metal nuts, bolts, or anything a magnet will stick to.
For those that are not aware, your basic computer hard drive contains at least one, if not several VERY strong magnets.
Strip the hard drive. Basically, just keep taking screws out until it falls apart. Find and remove the magnets around the base of the head. BE CAREFUL! some of them can be quite powerful, and I won't be responsible for any blood blisters that result. I have suffered nasty cuts when they snap together on my fingers. Although it is a great way to expand your (my) filthy ... filthy vocabulary.
Sometimes you get lucky, and the metal bracket the magnet is mounted to will be flat. If not, take two pairs of vice grips, or a bench vice and whatever (you get the idea) and clamp one on each end of the metal backing (not the magnet itself) and bend. The magnet should pop off. I have found it actually works much better if you can manage to keep the metal backing on, as it seems to concentrate the magnetic force to the other side.
place the magnet(s) under a your desk, or on the inside of a cabinet door. the magnetism is usually strong enough to radiate through to the top with no problem. arrange the nuts and bolts in impossible shapes. It's fun to play with during those long useless conference calls.
If you do this on a cabinet door, you can usually stack at least 4-5 nuts straight out before gravity takes over. It also comes in handy for holding papers and such.
You could also get or make a small wood platform and replicate the store bought toys by mounting the magnet underneath. Personally, I like the look of confusion until they figure it out. It also seems to make you do weird things. like slide your pen across the desk until it hits the invisible "wall".. I never get tired of that one.